If you wrote down all the numbers from 1 to 99, then how many times would you have written ‘7‘?
Total number in which 7 will occur at least once,is mentioned below,
7,17,27,37,47,57,67,77,87,97 - total 11 times 7 repeated.
70,71,72,73,74,75,76,78,79 - total 9 times 7 repeated
Hence total occurrence of 7 is 11+9=20
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