First i would come up with an equation that represents the question. the equation i would use is y=2x+40 with x being the number of visits and y being the total amount of money the membership would cost. then i would plug in the amount of $90 to get the equation 90=2x+40. then i would solve for x by first subtracting 40 from both sides to get 50=2x then divide by 2 on both sides to get x=25.
There are 8 choices for the first painting on the wall, 7 choices for the second, and so on, down to three choices for the sixth. Our result is thus 8*7*6*5*4*3=20,160. C is the correct answer.