Well, we can see that 115x refers to something that increases by 115 per purchase, while the 100 refers to a one time cost. Out of the possible answers the only one time charge is the installation charge, which refers to the 100$, and the 115x refers to the computers. In 115x, the 115 refers to how much a computer costs, and the x refers to the number of computers.
Cross multiply 3x-5 with 4 and 4x + 1 with 2
Which it would give you 12x-20=8x+2 then solve for x