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This puzzle should be easy, if you are careful. How many whole numbers less than 100 have 7 in them? 37 has a 7 in it, but 14 does not.
Answer: If you got almost the right answer, you did well. There are ten that are of this form: 7, 17, 27, ... 97. And there are ten of this form: 70, 71, 72, ... 79. That's 20 of them, right? Well 77 was in both lists, and we only want to count it once. So there are 19 of these numbers with 7 in them.
It is also possible to solve this by writing down all numbers from 1 to 100, and counting the ones with 7 in them. You should probably have figured out that there is an easier way. Well, now you know there is an easier way.
Another question: How many whole numbers less than 100 have 6 in them? This one may be even easier.