Round each number to the nearest 10. 33 rounds to 30 and 89 rounds to 90, thus in your head it's easier to mentally add 30 + 90 for purposes of estimation.
30 and 90... If the last number is 4 or lower than you round down... If the last number is 5 or higher than you round up... So the 3 in 33 is below four so you round down to 30 and the 9 in 89 is hight than 5 so therefore you round up to 90
