###Human: Frank wants to buy a new lamp for his bedroom. The cost of the cheapest lamp is $20, and the most expensive in the store is 3 times more expensive. How much money would Frank have remaining, if he currently has $90, and he buys the most expensive lamp available?, ### Assistant: The most expensive lamp cost 20 * 3 = $60.
So if Frank would pay for that lamp, he would leave himself with 90 - 60 = $30. 
 So the answer is :  30