howmanydifferentpositivefour-digitintegerscanbeformedhowmanydifferentpositivefour-digitintegerscanbeformedifthefirstdigitmustbe2,thelastdigitcannotbe0,anddigitsmayberepeated?翻译加解释谢谢答案是900个，解

Question

how many different positive four-digit integers can be formed
how many different positive four-digit integers can be formed if the first digit must be 2,the last digit cannot be 0,and digits may be repeated?翻译加解释谢谢
答案是900个，解释一下

Answer 1

如果第一个数字一定要2,最后一个数字一定要0,而数字可以重复,请问你可以组成多少组四位数的正数?
所以他要你找出头一个数字是2,最有一个数字是0,而中间两个数字可以重复的四位数正数的数目.也就是 2xx0 可以出城多少组数字.