51 is the highest multiple you can take from 53
One way to find this out is take 53-30 and you get 23 from this you know 21 is the highest multiple you can get so 30+21 = 51
51/3 is 17 so each student gets 17 blocks and the thus remainder is 2
Another way is to divide 30 and 21 by 3 and add them both together and you will get the same answer
Do always make the numbers easier to solve and save time scrambling..