Problem Solving

25 is halfway between 35 and 15..so A is 50%

Alligation probelm...

Answer : 50%

Allegation gives 1:1

So, 50 %

Alligation gives 1:1

So, 50 %

(D) 50%

(35*a+15*b)/(100*a+100*b)=25/100

a=b

i.e. a=b=1/2

askaichin wrote:Concrete mixture A is composed of 35 percent cement, 40 pecent sand and 25 percent gravel; concrete mixture B is composed of 15 percent cement, 30 percent sand and 55 percent gravel. If a mixture of A and B contains 25 percent cement, what percent of the weight of the mixture is A?

A 20%
B 33 1/3%
C 45%
D 50%
E 55 1/3%

100g of A has 35g of cement.
100g of B has 15g of cement.
35x + 15(1-x) = 25
35x + 15 - 15x = 25
20x = 10
x = 0.5 = 50%

Choose D

No need for calculations. Just Pick D because 25% is the average of 35% and 15% i.e. 50% of each of A and B.

Both mixtures are mixed in equal quantity.

Answer is B. 50%

askaichin wrote:Concrete mixture A is composed of 35 percent cement, 40 pecent sand and 25 percent gravel; concrete mixture B is composed of 15 percent cement, 30 percent sand and 55 percent gravel. If a mixture of A and B contains 25 percent cement, what percent of the weight of the mixture is A?

A 20%
B 33 1/3%
C 45%
D 50%
E 55 1/3%

Easy one for me , used the concept of Allegation explaining the concept with the diagram below...

Just Consider the Percentage of Cement in Mixture A and Mixture B.

GMATGuruNY's thought process is the exact one I used to solve this. If you write these out next to each other you can see that:

A = 35% cement, 40% sand, 25% gravel
B = 15% cement, 30% sand, 55% gravel

As you're looking for 25% cement, you would need an equal amount of A and B, as (35+15)/2 = 25. Thus the allocation is 50% for A and 50% for B.

Also a quick note: I would expect a person scoring regularly at 700+ level to solve this question without any writing/work at all.

Can someone post a similar problem? Im having a hard time understanding why everyone is saying using the average gives you the percent of A. What is the logic behind it? Is this not a ratio problem?

See here: https://www.beatthegmat.com/og-seed-mixture-t94289.html

Patrick_GMATFix wrote:Hi askaichin

To practice weighted avg questions in timed drills, set topic='Weighted Averages' and difficulty='600-700 & 700+' in the Drill Generator

How to get to the drill for the selected topics for GMAT

AndreiGMAT wrote:

Patrick_GMATFix wrote:Hi askaichin

To practice weighted avg questions in timed drills, set topic='Weighted Averages' and difficulty='600-700 & 700+' in the Drill Generator

How to get to the drill for the selected topics for GMAT

Can't speak for Patrick's, but we've got a free question bank searchable by type here.

Matt, thank you for reply.

I need to choose specifically "Weighted Averages" questions level 600-700 on this forum. If anyone can help me how to filter the questions on this forum , I will appreciate.

Thanks