A = lw
To isolate w, you have to divide l on both sides:
w = A/l
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.
Something along the lines of this...
