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# Need 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?

Apr 5, 2019

#1
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c is 9 is says it I think it is 9 that is what your question says

Apr 5, 2019
#2
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No offence but you're really not helpful.  I understand that any help I get at all is completly out of good will but still.  The question is what is c WHEN B EQAULS 3.

Apr 5, 2019
#3
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Oh I sorry let the proffesianals take over Nickolas  Apr 5, 2019
#4
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Its okay sorry if i was a little mean

Guest Apr 5, 2019
#5
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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?

$$a=kb\qquad \text{where k is a constant}\\ \text{When a=2, b=5 so}\\ 2=k*5\\ k=\frac{2}{5}=0.4\\ so\\ a=0.4b$$

$$a=\frac{m}{c}\qquad \text{where m is a constant}\\ \text{when a=2, c=9 }\\ 2=\frac{m}{9}\\ m=18\\ so\\ a=\frac{18}{c}\\ \text{sub in 0.4b for a}\\ 0.4b=\frac{18}{c}\\ when\; b=3\\ 0.4*3=\frac{18}{c}\\ 1.2=\frac{18}{c}\\ c=\frac{18}{1.2}\\ c=15$$

When b=3  c=15

You need to check for careless errors   :)

Apr 5, 2019