posted by Jacqueline on .
Find the number of Calories in a grapefruit and an orange.
(1) An orange has 15 more Calories than a grapefruit.
(2) Twenty oranges and ten grapefruit have 1800 Calories.
G = Grapefriut calories, R = oRange Calories
G = R - 15
20R + 10G = 1800
Substitute R - 15 for G and solve for R in second equation. Put that value in the first equation to find G. Check by putting both values in the second equation.