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# inverse proportion help!

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Suppose a is directly proportional to b, but inversely proportional to c. If a=2 when b=5 and c=9, then what is c when b=3?

Aug 7, 2019

#1
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$$a = p b\\ a = \dfrac q c\\ c = \dfrac{q}{pb}$$

$$2=5p \Rightarrow p=\dfrac 2 5\\ 2 = \dfrac q 9 \Rightarrow q = 18\\~\\ c = \dfrac{18}{\frac 2 5 \cdot 3} = 15$$

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Aug 7, 2019
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Thanks! :)

Guest Aug 9, 2019
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Suppose a is directly proportional to b, but inversely proportional to c. If a=2 when b=5 and c=9, then what is c when b=3?

$$a:b=2:5\\ a:c=9:2$$                    Nonsense. How blamable

$$b=\frac{5a}{2}\\ c=\frac{2a}{9}$$

b=3

$$a=\frac{2b}{5 }\\ a=\frac{2\cdot 3}{5}\\ a=\frac{6}{5}\\ c=\frac{2a}{9}=\frac{2\cdot6}{9\cdot 5}\\ \color{blue}c=\frac{4}{15}\ \color{black}Not\ correct.$$

!

Aug 7, 2019
edited by asinus  Aug 7, 2019
edited by asinus  Aug 8, 2019
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Asinus, you need to look up what inversely proportional means.

Melody  Aug 7, 2019
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Suppose a is directly proportional to b, but inversely proportional to c. If a=2 when b=5 and c=9, then what is c when b=3?

$$\color{BrickRed}a:b=2:5\\ \color{BrickRed}a:\frac{1}{c}=2:\frac{1}{9}\\ a=\frac{2b}{5}\\ \frac{2}{c}=\frac{a}{9}\\ c=\frac{18}{a}$$

b=3

$$a=\frac{2b}{5}\\ a=\frac{6}{5}\\ c=\frac{18}{a}\\ c=\frac{18\cdot 5}{6}\\ \color{blue}c=15$$

Thanks Melody! I have "looked up".

!

Aug 8, 2019