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Pls help me to solve this problem.(2) [#permalink]
31 Jul 2011, 02:32

00:00

A

B

C

D

E

Difficulty:

55% (hard)

Question Stats:

57% (03:47) correct
43% (03:05) wrong based on 7 sessions

The cost C of manufacturing a certain product can be estimated by the formular C=0.03rs(t^2), where r and s are the amounts, in pounds, oof the two major ingredients and t is the production time in hours. If r is increased by 50%, s is increased by 20%, and t is decreased by 30%, by approximately what percent will the estimated cost manufacturing the product change?

a) 40% increase b) 12% increase c) 4% incease d) 12% decease e) 24% decrease

Re: Pls help me to solve this problem.(2) [#permalink]
31 Jul 2011, 05:15

tracyyahoo wrote:

The cost C of manufacturing a certain product can be estimated by the formular C=0.03rs(t^2), where r and s are the amounts, in pounds, oof the two major ingredients and t is the production time in hours. If r is increased by 50%, s is increased by 20%, and t is decreased by 30%, by approximately what percent will the estimated cost manufacturing the product change?

a) 40% increase b) 12% increase c) 4% incease d) 12% decease e) 24% decrease

pls explain me, thank you. I need it.

From the formula we deduce that C directly proportional to r, s and t^2 hence an increase or decrease in these values will directly change the value of C by that factor

There new r => 1.5r new s => 1.2s new t^2 = (0.7t)^2 = .49t^2 let the constant 0.03 be termed as A

Putting these new values: C1 = A*1.5r*1.2s*.49t^2 C1 = 0.882A*r*s*t^2 = 0.882(C)

Re: Pls help me to solve this problem.(2) [#permalink]
31 Jul 2011, 14:19

Sudhanshuacharya wrote:

tracyyahoo wrote:

The cost C of manufacturing a certain product can be estimated by the formular C=0.03rs(t^2), where r and s are the amounts, in pounds, oof the two major ingredients and t is the production time in hours. If r is increased by 50%, s is increased by 20%, and t is decreased by 30%, by approximately what percent will the estimated cost manufacturing the product change?

a) 40% increase b) 12% increase c) 4% incease d) 12% decease e) 24% decrease

pls explain me, thank you. I need it.

From the formula we deduce that C directly proportional to r, s and t^2 hence an increase or decrease in these values will directly change the value of C by that factor

There new r => 1.5r new s => 1.2s new t^2 = (0.7t)^2 = .49t^2 let the constant 0.03 be termed as A

Putting these new values: C1 = A*1.5r*1.2s*.49t^2 C1 = 0.882A*r*s*t^2 = 0.882(C)

Re: Pls help me to solve this problem.(2) [#permalink]
31 Jul 2011, 14:21

arjun007 wrote:

Hi

How did you guys determine if it was an increase or decrease?

is it because the value of c > c1?

Hope it is clear to you how it is decreasing ... Taking a larger value like - 50 even decreased,so obviously a smaller value - 49 will always decrease _________________

Re: Pls help me to solve this problem.(2) [#permalink]
31 Jul 2011, 14:24

The reason for approximating is sometimes we make silly mistakes in calculation. So the best way is to pick smart numbers when needed.........Again it is up to you to decide the approach that works nest for you..... Some people can calculate this in head in (5-10) secs....then do not approximate...This is for the other folks who spend 20-30 secs on multiplication(calculation) _________________