SAT一道数学how many positive four-digit integers have 1 as their firs_数学解答

SAT一道数学
how many positive four-digit integers have 1 as their first digit and 2 or 5 as their last digit?
答案说是200,我为什么觉得是144?

人气：157 ℃　时间：2020-06-21 02:18:52

解答

也就是中间两位数的取法都有10种（从0取到9）,最后一位的取法有2种（取2或5）,那么,总的取法就是10*10*2=200
就是有200个不同的数字满足条件啦