分母是 50 以内（含 50 ）的自然数，且能化成有限小数的最简分数的分母有多少个？请一一写出．

How many denominators are there for simplest fractions where the denominator is a natural number less than or equal to 50 (inclusive) and which can be expressed as a finite decimal? Please list them all.

在 1 到 100 这 100 个正整数中任取其中的n个，要使这n个数中至少有一个合数，则n至少是多少？

If n numbers are chosen from the 100 positive integers between 1 and 100, what is the smallest possible value of n such that at least one of these n numbers is a composite number?

一个两位素数，将它的十位数字与个位数字对调后仍是一个两位素数，我们称它为＂美好素数＂，求所有＂美好素数＂之和。

A two-digit prime number is called a ‘beautiful prime’ if, when its tens digit and units digit are swapped, it remains a two-digit prime number. Find the sum of all ‘beautiful primes’.

Write the number that is 12 less than:(写出比下列数少12的数)

a 9   b -14  c -2   d 12