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Using cross multiplication:
1 8 + x 1 3 + x = 5 4
⇒ 5 ( 1 3 + x ) = 4 ( 1 8 + x )
6 5 + 5 x = 7 2 + 4 x
⇒ 7 2 − 6 5 = 5 x − 4 x
x = 7