Let the cost of 7 pencils be 'x' & the cost of 10 pencils be (7x + 15)/10
According to the Question,
⇒(7x + 15)/10 = x
⇒7x + 15 = x × 10
⇒7x + 15 = 10x
⇒7x - 10x = 15
⇒3x = 15
⇒x = 15/3
⇒x = 5
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