Formula
Profit = (selling price/cup - cost of making 1 cup)*number of cups - Cost of doing business.
Profit = (3.50 - 1.50)x - 100
Solve
Profit = 2x - 100
Now you need to make the profit = 0. That's your break even point.
0 = 2x - 100 Add 100 to both sides
100 = 2x Divide by 2
100/2 = x
x = 50
He has to sell 50 cups of coffee to break even.