Answer :
Cake=x
Pie=y
x+y=15
5x+7y=91
(x+y=15)-7
5x+7y=91
-7x+(-7y)=-105
5x+7y=91
-2x=-14
x=7
They sold 7 cakes
Pie=y
x+y=15
5x+7y=91
(x+y=15)-7
5x+7y=91
-7x+(-7y)=-105
5x+7y=91
-2x=-14
x=7
They sold 7 cakes