so we have 6+5+2 = 13
so any weight will have the following proportions:
if the weight is w, then
6/13 * w = the weight of apples
5/13 * w = the weight of peaches
2/13 * w = the weight of grapes
in other words, we just put the ratio of each fruit over the sum of the ratios, and multiply whatever weight we get.
so for 39 pounds of fruit
the weight of apples is 6/13 * 39 = 18
the weight of grapes is 2/13 * 39 = 6
so there are 18 - 6 = 12 more pounds of apples than grapes.
there was no need to multiply the ratios by 3 to get 18:15:6, that step was done implicitly in what i did