og11 diagnostic 13&15

13. If s and t are positive integers such that s/t=64.12, which of the following could be the remainder when s is divided by t?

答案是45

这题og的做法相当复杂， 是用列举法

15. The product of all the prime numbers less than 20 is closest to which of following powers of 10?

答案是10^7

这题og上是直接相乘起来的，

不知道各位有没有一些比较简便的方法来做这两道题？

第十三题，是怎么推导出the remainder after dividing s by t is s-64t?

解释中有个公式，看不懂，谁能解释一下呢？