Please help with the following question,

2) According to a recent study a 100% increase in the price of water for heavy users in Santa Cruz, California, caused the quantity of water they demanded to fall by average of 20%. (Before the increase, heavy users initially paid $1.55 per unit, but afterward they paid $3.10 per unit.) In percentage terms, how much did their water expenditure (price times quantity) which is the water company's revenue -- change? (Hint: See the Amazon example)