Question

When it is 1/5 full of water (by weight) a jug weights 560 grams. When it is 4/5 full, the jug weights 740 grams. What does the jug weigh when it is empty?

Answer 1

Difference between 1/5 full and 4/5 full is |560-740| grams.
That is 3/5 of capacity is 180 grams. Proportioned to full 5/5 of capacity,  grams. (5/3)180=300

Taking either of the given quantities, here trying 1/5 full,j+(1/5)300=560