So the problem I want to do-- there are many things you could do but I just picked one, and this is the one with round numbers, so I can do it without a calculator.

我想讲的这道题,类似的题目很多,但我就选了其中一个,这道题涉及的数字都是整数,所以我不需要计算器

耶鲁公开课 - 基础物理课程节选