Explanation:
let both unknowns be x and y
x = 5y. ......(1)
x + y = 90. ......(2)
sub eqn (1) into eqn (2)
5y + y = 90
6y = 90
y = 90/6
= 15
sub y = 15 to eqn (1)
x = 5y
x = 5 × 15
× = 75
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