第一，情景对话

A: I was wondering if you could give me a day off.

你能不能给我一天假？

B: What for?

为什？

A: I'll have to rewrite my thesis. That will take much time.

我得重新写论文，那要花很多时间。

B: Stop trying to fool me. I have your number.

别想糊弄我，我清楚你的底细。

第二，词海拾贝

have one’s number

exp. To understand or know someone or something well enough to anticipate what they will do. （心中有数，知道某人的底细）

e.g. Oh, don't worry — I won't let him cheat me. I've had his number.

哦，别担心，我不会让他欺骗我的。我清楚他的底细。

e.g. Oh, don't worry, I've had his number—he's a typical brat.

别担心，我非常了解他，他是个典型的小淘气。

e.g. I have his number. He enjoys less popularity than most other professors.

我知道他的底细，他不如其他的教授那样受欢迎。

第三，语汇积累

take much time：花费很多时间

What for? 为何？为什么？

give someone one day off：给某人一天的请假