Solution
For this case we can do the following:
Jake:
15x + 10y
Joshua:
10x + 12y
Where x= number of hours worked
y = number of parts sold
We assume that the amount earned by two are equal so we can do this:
15x + 10y= 10x+12y
Let's assume that both work the same amount of hours e.g 8
15*8 + 10y= 10*8 + 12y
Solving for y we got:
120- 80= 2y
y=40
They need to sell 40 gift cards in order to get the same amount of money if both work for 8 hours in a day