1. A box of thumbtacks weighed 1,590 grams. The box of thumbtacks weighed 1,470 grams after 40 thumbtacks were taken out from the box. How many thumbtacks were there in the box at first?
2. A big box can hold 12 marbles and a small box can hold 5 marbles. There are a total of 99 marbles. How many big boxes are there?
Find per unit wt of tack
(1590 - 1470) / 40 gm / tack
Now divide starting wt / unit weight to find tacks
gm / [(1590 - 1470) / 40] gm/tack = 530 tacks