Aschool bought several pairs of skis, each costing $40 and several pairs of skates, each costing $50. the cost of the whole purchase was $600. how many pairs of skates were purchased if there were 3 more pairs of skates than pairs of skis?

5 skis and 8 skates
y=ski
x=skates
x=y+3
40y+50x=600
40y+50(y+3)=600
y=5
x=8

Let the number of pair of skis and skates bought be x and y respectively. So that,
40x+50y = 600

There are 3 more pairs of skates. Therefore,
y-3 = x=> y=x+3
Substituting,

40x+50(x+3) =600 => 40x+50x+150=600 => 90x=600-150 = 450
x= 450/90 = 5 pairs of skis
y= x+3 =5+3 = 8 pairs of skates