How many 7's are there in 0-400?

Answer 1

80, here's how: First, just consider the numbers up to 99. Skip the 70's decade for now, and look at the other decades. 0-9, 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, 80-89, and 90-99 - there is one 7 in each of these 9 decades, so that's 9, now look at 70-79, there are 10 numbers, but there are 11 sevens (77 has two), so there are a total of 20 sevens in 0-99. For the 100's there are also twenty, and 200's 300's. The number 400 does not have any 7's so we have 4 centuries, each with 20 sevens, or 80 total.