An alloy weighing 24 ounces is 70 percent gold. How many ounces of pur

Question

An alloy weighing 24 ounces is 70 percent gold. How many ounces of pure gold must be added to create an alloy that is 90 percent gold?

A) 6
B) 9
C) 12
D) 24
E) 48

Source: Chili Hot GMAT

smyarga · Accepted Answer

Just want show the solution of such problems without unknown variables in three operations:) The main hint is to focus on the part that doesn't change!

Operation 1: Find the weight of part of alloy that doesn't change
30% of 24=0.3*24=7.2 ounces

Operation 2: Find the new weight of alloy. Since the weight of not gold part of alloy doesn't change after adding pure gold, 7.2 in new alloy is 10% of total weight.

7.2*10=72 ounces

Operation 3: Find what we add: 72-24=48 ounces of pure gold.

The correct answer is E.

PareshGmat · Answer

Answer = E) 48

Gold .......................... Not Gold ................. Total

16.8 .............................. 7.2 ........................ 24 (70% is gold. 30% is NOTGOLD)

16.8+x ............................ 7.2 ...................... 24+x (Say "x" gold is added to make it 90% gold. Means NOTGOLD will decrease to 10%)

Setting up the equation

\frac{10}{100}(24+x) = 7.2

x = 72-24 = 48

intheend14 · Answer

An alloy of 24oz which is 70% gold means we have 16.8 oz of gold. To get to something that is 90% gold, let's use this expression: (16.8 + x)/(24 + x) = 0.9 with x representing the amount of pure gold that must be added to get to 90%. The expression thus represents the new weight of gold over the total new weight of the alloy and this fraction should represent 90%. I was just surprised the answer was quite high--48oz... That's gonna get expensive. But that's the price you pay to have nice things, eh?

Choose E

Choose E

amitshri15 · Answer

In 24 ounces, gold is 24 * (70/100) = 16.8 ounces. Now we add x ounces of pure gold to make it 90% gold.
So 16.8 + x = (24 + x)* 90/100 => x =48.
Answer is E.

TeamGMATIFY · Answer

Initial: 24 ounces, 70 %gold or, 0.7*24 = 16.8 ounces gold and rest 7.2 other material.
Setp 1: Only gold is added. 
Let x ounces gold is added.
Gold: 16.8 + x, Other material = x

Final: We need 90% gold in the alloy.
Total weight = 24 + x

16.8 + x should be 90% of 24 + x
Hence, 16.8 + x = 0.9*(24 + x) 
0.1*x = 4.8
x = 48

Option E

KarishmaB · Answer

Back to weighted averages discussed here: https://www.gmatclub.com/forum/veritas- ... -mixtures/

w1/w2 = (100 - 90)/(90 - 70) = 1/2

Since 70% gold weighs 24 ounces, pure gold added should weigh twice of that so 48 ounces.

Answer (E)

gracie · Answer

let x=additional ounces of gold to be added
.7*24+x=.9(24+x)
x=48

singhabhijeet · Answer

We can use a direct formula for such questions:

Formula =  \frac{Solution * (Required Value - Present Value)}{(100 - Required Value)}

therefore, \frac{24*(90 - 70)}{(100 - 90)}

=>  \frac{24 * 20}{10} 
=> 24 * 2
=> 48

Ans: E

enidaddams · Answer

Can you provide more details, as the link is no longer functional.

An alloy weighing 24 ounces is 70 percent gold. How many ounces of pur

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