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If y=12 when x=15, what is x when y=21?

Sagot :

If y varies directly with x then

[tex]\begin{gathered} y=kx \\ \text{ Where k is a constant of a variation} \end{gathered}[/tex]

First, we need to find the constant of a variation k, for this, we use the given values of x and y:

[tex]\begin{gathered} y=kx \\ 12=k\cdot15 \\ \text{ Divide by 15 from both sides of the equation} \\ \frac{12}{15}=\frac{k\cdot15}{15} \\ \frac{12}{15}=k \\ \text{ Simplifying} \\ \frac{3\cdot4}{3\cdot5}=k \\ \frac{4}{5}=k \end{gathered}[/tex]

Then since we already have the value of k we can find the value x when y = 21:

[tex]\begin{gathered} y=kx \\ 21=\frac{4}{5}x \\ \text{ Multiply by 5 from both sides of the equation} \\ 5\cdot21=5\cdot\frac{4}{5}x \\ 105=4x \\ \text{ Divide by 4 from both sides of the equation} \\ \frac{105}{4}=\frac{4x}{4} \\ \frac{105}{4}=x \end{gathered}[/tex]

Therefore, if y = 12 when x = 15, then

[tex]x=\frac{105}{4}[/tex]

when y = 21.

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