Pria sits at the desk by the window in Room Nine. She stares at the first page of her maths exam. She is forty-three. She works nights at a warehouse, and she has studied for this paper every morning for a year. Her pencil case is on the desk. Her hands are cold.
普丽娅坐在九号教室靠窗的课桌前，盯着数学试卷的第一页。她四十三岁。她在仓库上夜班，为了这场考试，她每天早晨都学习，已经学了一年。她的文具盒放在桌上。她的双手冰凉。
The invigilator walks slowly between the rows. She is young, and her name badge says Lotte. She holds a stack of spare papers as if they might bite her. Pria guesses it is her first time.
监考老师在一排排课桌之间慢慢走动。她很年轻，胸牌上写着洛特。她抱着一叠备用试卷，好像它们会咬人。普丽娅猜这是她第一次监考。
"You may begin," says Lotte. Her voice cracks on the last word.
“可以开始了，”洛特说。她的声音在最后一个字上破了音。
Pria begins. The first question is easy, and so is the second. By the fifth, she feels something she has not felt in a classroom for twenty years. She feels clever.
普丽娅开始答题。第一道题很简单，第二道也是。做到第五道时，她感受到了二十年来在教室里不曾有过的感觉。她觉得自己很聪明。
At twenty past seven, a small sound comes from the door. Lotte is whispering with a man in a grey suit. He has a lanyard and a tired face. He points towards the window, and then he leaves. Lotte comes back down the aisle with her eyes fixed on the floor.
七点二十分，门口传来一点小动静。洛特正和一个穿灰西装的男人低声说话。他挂着工作牌，面带倦容。他朝窗户的方向指了指，然后离开了。洛特沿着过道走回来，眼睛盯着地面。
She stops beside Pria's desk and bends down. "Sorry," she whispers. "I cannot talk during the exam. But could you look outside? Only look. I am not allowed to ask about the paper."
她在普丽娅的课桌旁停下，弯下腰。“对不起，”她低声说，“考试期间我不能说话。可是你能往外面看一眼吗？只是看一眼。我不能问试卷的事。”
Pria frowns. Then she lifts her head and looks through the window.
普丽娅皱起眉头。然后她抬起头，透过窗户往外看。
Below, on the pavement outside the main door, a large drawing covers the paving stones. It is made of white chalk. Pria sees a square, and inside it a smaller grid of nine boxes. Each box holds a number, except the middle one. The middle one is empty.
下面，在正门外的人行道上，一幅大画铺满了铺路石。它是用白粉笔画的。普丽娅看到一个方框，里面是一个由九个小格组成的小方阵。每个格子里都有一个数字，只有正中间那一格是空的。
The numbers are eight, one, six in the top row. Then three, blank, seven. Then four, nine, two.
第一行是八、一、六。第二行是三、空格、七。第三行是四、九、二。
Pria looks at it for a few seconds. Then she looks back at Lotte and nods once. She returns to her exam, because there is still a lot of paper left. But her mind keeps drifting down to the pavement.
普丽娅看了几秒钟，然后回头看向洛特，点了一下头。她回到试卷上，因为还有很多题没做完。可她的思绪总是飘到下面的人行道上。
For the next hour, the room is silent. Pria finishes question fourteen. She checks her working, and then she checks it again. At eight twenty, she turns over her last page and puts down her pencil. There are still thirty minutes left, and the rules say she must stay in her seat.
接下来的一个小时，教室里一片寂静。普丽娅做完了第十四题。她检查了演算过程，又检查了一遍。八点二十分，她翻过最后一页，放下铅笔。还剩三十分钟，规定她必须留在座位上。
So she sits and looks out of the window. The pavement drawing is clearer from up here. Whoever made it drew each line with great care. The digits are neat.
于是她坐着，望向窗外。从这里看，人行道上的画更清楚了。不管是谁画的，每一条线都画得很用心，数字也很工整。
She does the sum without trying. Eight plus one plus six is fifteen. The first column adds up to fifteen as well. Every row, every column and both diagonals should be fifteen. It is a magic square, and the missing number is five.
她不费力气就算了出来。八加一加六是十五。第一列加起来也是十五。每一行、每一列和两条对角线，加起来都该是十五。这是一个幻方，缺的数字是五。
At eight fifty, a bell rings. The students stand, stretch and hand in their papers. Lotte collects them one by one, and she counts each pile twice. When Pria gives hers over, Lotte holds on to it a moment too long.
八点五十分，铃响了。学生们站起来，伸伸腰，交上试卷。洛特一份一份地收，每一叠都数了两遍。普丽娅交卷时，洛特握着卷子的时间稍微长了一点。
"Thank you," Lotte says quietly. "Please wait outside the door. I need to ask you something."
“谢谢你，”洛特轻声说，“请在门外等一下。我想问你点事。”
In the corridor, Pria leans against the wall. Her legs feel weak. She has finished. Whatever the result, she has finished.
走廊里，普丽娅靠着墙。她双腿发软。她考完了。不管结果如何，她考完了。
Lotte comes out two minutes later with her bag. Her face looks pale.
两分钟后，洛特拎着包出来了。她的脸色有些苍白。
"I am so sorry," she says. "You have just done a two-hour exam, and now I want a favour."
“真对不起，”她说，“你刚考完两个小时的试，我却还要请你帮忙。”
"What is the problem?" asks Pria.
“怎么了？”普丽娅问。
"The head of department thinks the drawing is a code." Lotte talks fast. "He thinks someone is signalling answers to the students. Or it is a joke about the exam. He wants to know who did it before the results go out. He wants me to find out." She lowers her voice. "I teach history. I do not understand any of it."
“系主任觉得那幅画是密码，”洛特说得很快，“他觉得有人在给学生传答案。或者是在拿这场考试开玩笑。他想在成绩公布前查出是谁干的。他要我去查。”她压低了声音。“我是教历史的。我一点也看不懂。”
Pria almost laughs. "It is not a code," she says. "It is a magic square. The missing number is five."
普丽娅差点笑出来。“那不是密码，”她说，“那是个幻方。缺的数字是五。”
"How do you know that?"
“你怎么知道？”
"Every row adds up to fifteen. Look." Pria takes a pen from her pocket and writes the numbers on her hand. "Eight, one, six. Three, five, seven. Four, nine, two. Fifteen, fifteen, fifteen. It is an old puzzle. Children learn it at school."
“每一行加起来都是十五。你看。”普丽娅从口袋里拿出一支笔，把数字写在手上。“八、一、六。三、五、七。四、九、二。十五，十五，十五。这是个老谜题，小孩子在学校里都学过。”
Lotte stares at the numbers. Then she lets out a long breath. "So it is harmless."
洛特盯着那些数字。然后她长长地呼出一口气。“这么说它没有恶意。”
"It is harmless," says Pria. "But somebody still drew it, and I would like to know who."
“没有恶意，”普丽娅说，“不过总归有人画了它，我想知道是谁。”
They walk down the stairs together. The lobby is full of tired students and the smell of floor polish. At the main door, Pria stops, because the drawing looks different from ground level. There is something she did not see from above. At the bottom corner of the square, a tiny arrow points to the left.
她们一起下楼。大厅里挤满了疲惫的学生，空气里有地板蜡的味道。走到正门口，普丽娅停下了脚步，因为从地面上看，这幅画和从楼上看不一样。有一样东西，她从上面没有看见。方阵的下角，有一个小小的箭头指向左边。
They follow it. On the next paving stone, there is a second drawing. It is a small spiral with numbers along its curve. It is a different puzzle, and it is also unfinished.
她们顺着箭头走。下一块铺路石上有第二幅画，是一个小小的螺旋，沿着曲线写着数字。那是另一个谜题，同样没有完成。
"There are more," whispers Lotte.
“还有呢，”洛特低声说。
The arrows lead them along the building wall, round the corner and towards the bins. Each stone holds a new puzzle. There is a triangle of numbers and a clock face with no hands. There is a little ladder with missing steps. Pria feels like a girl again.
箭头带着她们沿着教学楼的墙，绕过拐角，走向垃圾桶。每块石头上都有一个新谜题。有一个数字三角形，有一个没有指针的钟面，还有一架缺了几级的小梯子。普丽娅觉得自己又变回了小女孩。
At the end of the path, a man in a blue overall is kneeling beside the back door. He holds a stub of chalk and draws a careful curve. A bucket and a mop stand beside him.
小路尽头，一个穿蓝色工作服的男人跪在后门旁边。他握着一截粉笔，正小心地画一条曲线。旁边放着一只水桶和一把拖把。
"Good evening," says Pria.
“晚上好，”普丽娅说。
The man looks up and does not seem surprised. He is about sixty, and he has chalk dust on his knees. "Good evening," he says. "Are you here for the next one?"
男人抬起头，似乎并不惊讶。他大约六十岁，膝盖上沾着粉笔灰。“晚上好，”他说，“你们是来找下一道题的吗？”
"The next one?" asks Lotte.
“下一道题？”洛特问。
"I draw one every night during exam week," he says. "The students come out of the building looking so serious. I thought they might like something to solve on the way home." He glances at Lotte's badge. "I hope I have not made trouble."
“考试周的每个晚上，我都画一道，”他说，“学生们从楼里出来的时候，神情那么严肃。我想，他们也许愿意在回家路上有点东西可以解。”他瞥了一眼洛特的胸牌。“我希望没有惹麻烦。”
Lotte opens her mouth and closes it again. "My head of department thought you were sending hidden messages."
洛特张了张嘴，又闭上了。“我们系主任以为你在传递暗号。”
"Only the messages that say, think about this," says the man, and he smiles.
“只传递那种暗号：想想这个，”男人说着，笑了。
Pria crouches beside him and studies the half-finished curve. "You were a teacher once," she says.
普丽娅在他身边蹲下，端详着那条没画完的曲线。“你以前是老师，”她说。
"I taught maths for thirty years," he answers. "Then my knees got old, and now I mop floors. But I miss it every day." He taps the paving stone. "This one is hard. Nobody has solved the first half yet."
“我教了三十年数学，”他回答，“后来膝盖老了，现在我拖地。可我每天都想念它。”他敲了敲铺路石。“这一道很难。前半部分还没有人解出来过。”
Pria takes the chalk. She thinks for a moment. Her exam is finished, her shift starts in two hours, and she should go home and sleep. But the puzzle looks at her like a door that has been left open.
普丽娅接过粉笔。她想了片刻。考试结束了，她两个小时后要上班，她应该回家睡觉。可这个谜题看着她，像一扇没有关上的门。
"Seven," she says at last, and she writes it in the gap.
“七，”她终于说，把它写进了空格里。
The man leans closer. He reads the numbers and then reads them again. His eyebrows go up. "That is right," he says. "That is exactly right. Most people never see it."
男人凑近了些。他读着那些数字，又读了一遍。他的眉毛扬了起来。“对了，”他说，“完全正确。大多数人都看不出来。”
Lotte lets out a small laugh. She sits down on the kerb, holds her bag in her lap and looks at both of them. The day has been very long for her.
洛特轻轻笑了一声。她在路沿上坐下，把包抱在腿上，看着他们两个。对她来说，这一天实在太漫长了。
"So what do I tell my head of department?" she asks.
“那我该怎么跟系主任说？”她问。
"Tell him the truth," says Pria. "A caretaker drew a puzzle, and a student solved it."
“告诉他实情，”普丽娅说，“一位管理员画了个谜题，一个学生解出来了。”
"He will want a name," says Lotte.
“他会要名字的，”洛特说。
"My name is not important," the man says. He picks up his bucket. Then he looks at Pria, and there is a bright, curious light in his eyes. "But I am running out of easy puzzles. Do you know any hard ones?"
“我的名字不重要，”男人说。他拎起水桶。然后他看着普丽娅，眼睛里闪着明亮的好奇。“不过我的简单谜题快用完了。你知道什么难的吗？”
