Divide 750 by 35. Note that 5×7=35. To divide 750 easily, divide by 750 by 5 first: 750/5=150.
Now we divide 150 by 7: 150/7=21 remainder 3. So we need 21 boxes plus another for the remainder of the tins=3×5=15 tins. (We need 22 boxes, which could contain 770 tins, whereas 21 boxes would only hold 770-35=735 tins.)