A pail with 40 washers in it weighs 500 grams. The same pail with 20 washers in it weighs 420 grams. How much does the pail weigh?

The pail weighs 340 grams

The difference between the two pails is 20 washers meaning that the loss of 80 grams is equivalent to the difference, so if 20 washers is equal to 80 grams then 2 times the number of washers (40 washers) would equal to 160 grams (total mass of washers). Once you know the weight of the total mass of the washers you'll be able to solve for x. x+160=500. Subtract 160 from both sides: x+160-160=500-160 Leaving you with the mass of the pail. x=340