Assuming you're interested in the number of each that were sold, the system of equations will express the relation for total sold, and the relation for total revenue. We have used the variable c and p to represent numbers of cakes and pies sold, respectively. Your choices may use x and y, instead.
c + p = 10
7c + 6p = 66
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The solution can be found by subtracting 6 times the first equation from the second. That tells you 6 cakes were sold, hence 4 pies.
