Since we are given a ratio/proportion problem and 3 values and 1 variable then we can use the Rule of three to solve this problem. We simply multiply the diagonal values available and divide by the 3 value to find the variable of the ratio like so...
4 pairs <======> $90.52
d pairs <======> $135.78
(4 * 135.78) / 90.52 = 6 pairs
Finally, we can see that Justin ended up buying a total of 6 pairs of sneakers.
