Question

Marcos is 7 years older than his sister, Rosa. The sum of their ages is 1 less than 3 times Rosa's age. How old is each?
m = r + 7

m + r + 1 = 3 r
so m = 2 r - 1

so
r+7 = 2 r - 1
8 = 1 r

Rosa is 8

m = 8 + 7 = 15

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