Question

the number 12345678910111213....979899 consists of all the integers from 1 through 99 written back to back. that's a very big number! is it a multiple of 9? explain.

Answer 1

It's a multiple of 9 if the sum of all the digits is a multiple of 9. So, add all the digits: there are 20 of each (if you include 0). So, the sum is 20*(1+2+3+4+5+6+7+8+9) = 20*45, which is a multiple of 9. So, your number is a multiple of 9.