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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.

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