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Question Number 106665 by deep last updated on 06/Aug/20

 Find a fourth proportional to   3, 12 and 15

$$\:\mathrm{Find}\:\mathrm{a}\:\mathrm{fourth}\:\mathrm{proportional}\:\mathrm{to} \\ $$$$\:\mathrm{3},\:\mathrm{12}\:\mathrm{and}\:\mathrm{15} \\ $$

Answered by nimnim last updated on 06/Aug/20

Let x be the fourth proportional.  ∴      3:12::15:x  ⇒3x=12×15  ⇒x=((12×15)/3)=60

$${Let}\:{x}\:{be}\:{the}\:{fourth}\:{proportional}. \\ $$$$\therefore\:\:\:\:\:\:\mathrm{3}:\mathrm{12}::\mathrm{15}:{x} \\ $$$$\Rightarrow\mathrm{3}{x}=\mathrm{12}×\mathrm{15} \\ $$$$\Rightarrow{x}=\frac{\mathrm{12}×\mathrm{15}}{\mathrm{3}}=\mathrm{60} \\ $$

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