At a certain fast-food restaurant, Brain can buy 3 burgers, 7 shakes and one order of Fries for $120 exactly. At the same place it would cost $164.50 for 4 burgers, 10 shakes and one order of fries. How much would it cost for an ordinary meal of one burger, one shake and one order of fries?
Answer
If b stands for burgers, S for shakes and f for fries
3b + 7s + f = 120
4b + 10s + f = 164.50
solving these two we get
b + 3s = 44.50
rewriting 3b +7s + f as
2b + b + 7s + f = 120
2(44.50 – 3s)+ b + 7s + f = 120
89 -6s + b + 7s + f = 120
b + s + f = 120 – 89 = 31