sat数学.题目看不懂..

题目

sat数学.题目看不懂..
Tim wrote a seven-digit phone number on a piece of paper.He later tore the paper accidentally and the last two digit were lost．What is the maximum number of arrangements of two digits,using the digits 0 through 9,that he could use in attempting to reconstruct the correct phone number?

答案

Tim在一张纸上记下了一个七位数的电话号码.后来他不小心撕破了这张纸,号码的最后两位数弄没了.问用数字0到9所能组成的两位数的组合最多有多少,使他能够用来恢复正确的电话号码?
答案肯定是100.0到9任选两个的组合,简单的说就是10x10=100.用公式就是P(1 10)xP(1 10)=100.如果用罗列的办法可知,（00~09）、（10~19）、...(80~89)、（90~99）,一共100种组合.