7. When a jar is one-fifth filled with water, it weighs $560 \mathrm{~g}$. When the same jar is four-fifths filled with water, it weighs $740 \mathrm{~g}$. What is the weight of the empty jar?
A $60 \mathrm{~g}$
B $\quad 112 \mathrm{~g}$
C $180 \mathrm{~g}$
D $300 \mathrm{~g}$
E $500 \mathrm{~g}$
Solution
E

The information in the question tells us that the amount of water that would fill three-fifths of the jar weighs $(740-560) \mathrm{g}=180 \mathrm{~g}$. Therefore the weight of water that would fill one-fifth of the jar is $(180 \div 3) \mathrm{g}=60 \mathrm{~g}$. Hence the weight of the empty jar is $(560-60) \mathrm{g}=500 \mathrm{~g}$.