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If one coupons costs you 3 salads, 10 salads costs you 10 × 3 = 30 salads.
To get $10 in coupons, the least amount of salads you would need to buy is 30.
x - y = 3
x = 3 + y
x -3 = y
y = x-3
put it in 1st equation
2x +y = 15
2x + x - 3 = 15
3x - 3 = 15
3x = 15+3 = 18
x = 18/3 = 6
x = 6
