Laryssa buys $40$ litres of gasoline a week at $$1\mathrm{.}00$ per litre. When the price rose to $$1\mathrm{.}50$ last summer, she bought only $30$ litres a week. Because this $50\%$ increase in price caused a $25\%$ reduction in quantity demanded, Laryssa's demand isand her total expenditure $($P x Q$)$ last summer