An alloy contains tin and copper in the ratio of 4:5 . If tin has 20%

Question

An alloy contains tin and copper in the ratio of 4:5 . If tin has 20% impurity and copper has 58% , what is the avg impurity percentage ?

A. 20%

B. 41.1%

C. 35%

D. 38%

E. 43.1%

Bunuel · Answer

An alloy contains tin and copper in the ratio of 4:5 . If tin has 20% impurity and copper has 58% , what is the avg impurity percentage ?

A. 20%

B. 41.1%

C. 35%

D. 38%

E. 43.1%

(4*0.2 + 5*0.58)/(4+5) = ~41%.

Answer: B.

PareshGmat · Answer

Impurity in tin = 20% of 4/9 = 80/900
Impurity in Cu = 58% of 5/9 = 290/900

Total Impurity = 380/900. Percent = 380/9 = 41.2
Answer = B

OptimusPrepJanielle · Answer

Tin and copper are in ratio of 4:5. So let total mixture be 4+5 = 9. So, tin = 4 and copper = 5. Now, tin has 20% impurity = 20% * 4 = 0.8 Also, copper has 58% impurity = 58% * 5 = 2.9 Average = (0.8 + 2.9)/(4+5) = 3.7/9 = 0.411 = 41.1% Hence option (B). -- Optimus Prep's GMAT On Demand course for only $299 covers all verbal and quant. concepts in detail. Visit the following link to get your 7 days free trial account: https://www.optimus-prep.com/gmat-on-demand-course

vijayshree · Answer

An alloy contains tin and copper in the ratio of 4:5 . If tin has 20% impurity and copper has 58% , what is the avg impurity percentage ?

a) 20%

b) 41.1%

c) 35%

d) 38%

e) 43.1%

The problem is on ratio / Proportion but look closely it can be solved with  weighted average concept

(4*0.2 + 5*0.58)/(4+5) = ~41%.

Answer: B.

BD0176 · Answer

Its a 10sec Question. 
Because tin:Copper is in the ratio 4:5, there is more of Copper and hence more impurities from Copper in the final alloy. Since Copper has 58% impurities, the overall average should be closer to 58%, which narrows down the options to B or E. Since T:C is close to 1 (4:5), we want to pick the option closer to the middle but more towards C, which is 42%. Hence option B

rohit8865 · Answer

Can be done with weighted avg. formula
w1/w2=(a2-avg)/(avg-a1)
4/5=(58-x)/(x-20)
x=41.1
Ans B

GMAT2645 · Answer

Perhaps this is extremely obvious, but how are you able to "see" the last operation so precisely. I.e. does it not require no division by a decimal without a calculator to determine the ~41%? Best regards.

Bunuel · Answer

\frac{(4*0.2 + 5*0.58)}{(4+5)} = \frac{0.8+2.9}{9}= \frac{3.7}{9} .

3.6/9 = 0.4, so 3.7/9 will be a bit larger.

Masterscorp · Answer

We can also transition the fraction to percent by recognizing that a fraction with denominator of 9 can be multiplied by 11 to generate an approximation.

\frac{3.7}{9}*11 = \frac{40.7}{99} . As we divide only by 99 and not by 100 the value in percent must be slightly higher so 41,1% is the only reasonable answer.

ScottTargetTestPrep · Answer

Let’s re-express the ratio of tin to copper as 4x : 5x. The average impurity is:

/9x = (0.8x + 2.9x)/9x = 3.7/9 = 0.411 = 41.1%.

Answer: B

GMAT2645 · Answer

Are you completing the (4*0.2 + 5*0.58)/(4+5) = ~41% calculation in your head? Thank you.

GMAT2645 · Answer

When it comes to the simple arithmetic such as "5x(0.58) = 2.9x" or "3.7/9 = 0.411" what shortcuts are you using to compute these calculations? Thank you.

Bunuel · Answer

Check this: https://gmatclub.com/forum/an-alloy-con ... l#p2058527

bumpbot · Answer

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An alloy contains tin and copper in the ratio of 4:5 . If tin has 20%

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