If y varies directly with x, and If y = 14 when x = 5, how do you find x when y = 28?

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If y varies directly with x, and If y = 14 when x = 5, how do you find x when y = 28?

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Answer 1 · 2016-07-24T17:08:35

10

Explanation:

$y prop x implies y = kx$ where $k$ is a proportionality constant.

Can do this in two ways.

$color(blue)("Method One")$

$y = kx implies k = y/x = 14/5$

$x = y/k = 28/(14/5) = 10$

$color(blue)("Method Two")$

$y_1 = kx_1 and y_2 = kx_2$

$y_2/y_1 = (kx_2)/(kx_1) = x_2/x_1$

$implies x_2 = x_1y_2/y_1$

$x_2 = 5*(28)/(14) = 10$

Although the second method seems more effort just now, if you progress in maths or especially if you do physics you'll quickly find that formulae can have a lot of variables and a quick way to figure out the effects of changing one of them is to use ratios as shown in the second method.

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If y varies directly with x, and If y = 14 when x = 5, how do you find x when y = 28?

1 Answer

Explanation:

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