Of the two-digit positive integers less than 20 how many have dif_数学解答

Of the two-digit positive integers less than 20 how many have different digits
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人气：311 ℃　时间：2020-01-28 23:17:41

解答

小于20的两位数的正整数中,个位和十位上的数字不同的数字有几个?