How many numbers between 2000 to 7999 contain at least one 5?

Answer 1

for temporary, pretend it starts with ZER0...

They are 5,15,25,35,45,50-59,65,75,85,95

so for each block of 100, there are 18

then for each block of 1000, there are 18*10 = 180

there are 6 such blocks of 1000: 2000-2999, 3000-3999, ... ,7000-7999

(that is 2000s, 3000s, 4000s, 5000s, 6000s, 7000s)

180*6 = 1080

you can now apply the same reasoning and patterns to the similar problem

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