Hello :
3/8 = x/(15 + x)
⇔ 3(15 + x)/8 = x
⇔ (45 + 3x)/8 = x
⇔ 45 + 3x = 8x
⇔ 8x - 3x = 45
⇔ 5x = 45
⇔ x = 45/5
⇔ x = 9
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