Question

Problem How many numbers between 111 and 100100100 (inclusive) are divisible by 101010 or 777?

Answer 1

I suspect you meant

"How many numbers between 1 and 100 (inclusive) are divisible by 10 or 7?"

• Count the multiples of 10:

⌊100/10⌋ = ⌊10⌋ = 10

• Count the multiples of 7:

⌊100/7⌋ ≈ ⌊14.2857⌋ = 14

• Count the multiples of the LCM of 7 and 10. These numbers are coprime, so LCM(7, 10) = 7•10 = 70, and

⌊100/70⌋ ≈ ⌊1.42857⌋ = 1

(where ⌊x⌋ denotes the "floor" of x, meaning the largest integer that is smaller than x)

Then using the inclusion/exclusion principle, there are

10 + 14 - 1 = 23

numbers in the range 1-100 that are divisible by 10 or 7. In other words, add up the multiples of both 10 and 7, then subtract the common multiples, which are multiples of the LCM.