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Intern  Joined: 31 Aug 2012
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A chemist combined 'a' milliliters of a solution that  [#permalink]

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Difficulty:   45% (medium)

Question Stats: 61% (01:31) correct 39% (02:01) wrong based on 410 sessions

HideShow timer Statistics A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?

(1) b = 20
(2) c = 24
Magoosh GMAT Instructor G
Joined: 28 Dec 2011
Posts: 4488
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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Daeny wrote:
A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?
(1) b = 20
(2) c = 24

I'm happy to help with this. Before we look at the statements, let's process what's in the prompt.

First of all, the total amount of liquid "a" mL of one plus "b" mL of the other make "c" mL of the result. This must mean:

(A) a + b = c

That's one equation, the volume equation, that relates the three variables. Now, consider the concentration of the substance S.
The first solution was "a" mL, and it was 20% S, so that contained 0.20*a mL of S
The second solution was "b" mL, and it was 8% S, so that contained 0.08*b mL of S.
Those two were combined.
The result solution was "c" mL, and it was 10% S, so that contained 0.10*c of S
The concentration equation summarizes this:
(B) 0.20*a + 0.08*b = 0.10*c

That's two unknowns and three equations. We currently couldn't solve for values, but if we got down to two unknowns with two equations, then we could solve for everything. Whenever (# of variables) is less then or equal to (# of equations), as long as the equations are truly different (independent), then you can solve for values.

Now, look at the DS statements. Each one gives a value for one of the three variables. If any one of the variables gets a value, then we are down to two unknowns with two equations, and we can solve for everything. Each DS statement individually allows me to solve for everything and answer the prompt question definitively. Therefore, both statements are individually sufficient, and answer = D.

Notice how little actually algebra-math I had to do once my equations were set up. This is the point of DS question --- you often don't have to do a lot of calculations, and in fact, it's often a mistake to do so. In fact, if you already knew about the two equations --- the volume equation and the concentration equation --- then you could have jumped directly to the deductions without having done any algebra-math at all. That would be a very elegant way to dispatch this problem!

Does all this make sense? Please let me know if you have any further questions.

Mike _________________
Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)
General Discussion
Intern  Joined: 25 Sep 2012
Posts: 5
Location: Italy
GMAT 1: 590 Q38 V32 GMAT 2: 700 Q47 V39 GPA: 3.91
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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mikemcgarry wrote:
Daeny wrote:
A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?
(1) b = 20
(2) c = 24

I'm happy to help with this. Before we look at the statements, let's process what's in the prompt.

First of all, the total amount of liquid "a" mL of one plus "b" mL of the other make "c" mL of the result. This must mean:

(A) a + b = c

That's one equation, the volume equation, that relates the three variables. Now, consider the concentration of the substance S.
The first solution was "a" mL, and it was 20% S, so that contained 0.20*a mL of S
The second solution was "b" mL, and it was 8% S, so that contained 0.08*b mL of S.
Those two were combined.
The result solution was "c" mL, and it was 10% S, so that contained 0.10*c of S
The concentration equation summarizes this:
(B) 0.20*a + 0.08*b = 0.10*c

That's two unknowns and three equations. We currently couldn't solve for values, but if we got down to two unknowns with two equations, then we could solve for everything. Whenever (# of variables) is less then or equal to (# of equations), as long as the equations are truly different (independent), then you can solve for values.

Now, look at the DS statements. Each one gives a value for one of the three variables. If any one of the variables gets a value, then we are down to two unknowns with two equations, and we can solve for everything. Each DS statement individually allows me to solve for everything and answer the prompt question definitively. Therefore, both statements are individually sufficient, and answer = D.

Notice how little actually algebra-math I had to do once my equations were set up. This is the point of DS question --- you often don't have to do a lot of calculations, and in fact, it's often a mistake to do so. In fact, if you already knew about the two equations --- the volume equation and the concentration equation --- then you could have jumped directly to the deductions without having done any algebra-math at all. That would be a very elegant way to dispatch this problem!

Does all this make sense? Please let me know if you have any further questions.

Mike Thank you! it was extremely clear SVP  Joined: 06 Sep 2013
Posts: 1651
Concentration: Finance
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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Daeny wrote:
A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?

(1) b = 20
(2) c = 24

Yeah as Mike said we have the following

0.2a + 0.08b = 0.1c
2a + 8b = c

Let's go with statement 1 first

b = 20

Since we are given the point values and result we can find the weight between 'a' and 'b' by applying differentials

That is 10a - 2b = 0
10a = 2b
5a = b

Sufficient

Statement 2

c= 24

a + b = 24

We already have 5a = b

So we can solve again

D here

Is this clear?

Cheers!
J Manager  Joined: 04 Jan 2014
Posts: 101
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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I don't get it. How can we derive the exact value of a by knowing just c? Can anyone explain further?
Math Expert V
Joined: 02 Sep 2009
Posts: 55804
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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pretzel wrote:
I don't get it. How can we derive the exact value of a by knowing just c? Can anyone explain further?

A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?

a milliliters and b milliliters produce c milliliters --> a + b = c.
20% of S in a milliliters and 8% of S in b milliliters give 10% in c milliliters: 0.2a + 0.08b = 0.1c.

(1) b = 20 --> we have a + 20 = c and 0.2a + 0.08*20 = 0.1c. We have two distinct linear equations with two unknowns (a and c), so we can solve for both. Sufficient.

(2) c = 24 --> the same here: a + b = 24 and 0.2a + 0.08b = 0.1*24. We have two distinct linear equations with two unknowns (a and b), so we can solve for both. Sufficient.

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Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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jlgdr wrote:
Daeny wrote:
A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?

(1) b = 20
(2) c = 24

Yeah as Mike said we have the following

0.2a + 0.08b = 0.1c
2a + 8b = c

Let's go with statement 1 first

b = 20

Since we are given the point values and result we can find the weight between 'a' and 'b' by applying differentials

That is 10a - 2b = 0
10a = 2b
5a = b

Sufficient

Statement 2

c= 24

a + b = 24

We already have 5a = b

So we can solve again

D here

Is this clear?

Cheers!
J It should be
0.2a + 0.08b = 0.1c
2a + 0.8b = c
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Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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Daeny wrote:
A chemist combined 'a' milliliters of a solution that contained 20 percent substance S, by volume, with 'b' milliliters of a solution that contained 8 percent substance S, by volume, to product 'c' milliliters of a solution that was 10 percent substance S, by volume. What is the value of a?

(1) b = 20
(2) c = 24

Find my solution attached.

Cheers,
Attachments del2.PNG [ 24.05 KiB | Viewed 1506 times ]

Intern  B
Joined: 29 Apr 2017
Posts: 29
Location: India
Concentration: Finance, Accounting
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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Some help here....I guess I did not comprehend the question well.

When I initially read the question, I could not make out what the question is asking; i.e. I did not understand (yhe bold part) that a is combined with b to form c.
Intern  B
Joined: 29 Apr 2017
Posts: 29
Location: India
Concentration: Finance, Accounting
Re: A chemist combined 'a' milliliters of a solution that  [#permalink]

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Some help here....I guess I did not comprehend the question well.

When I initially read the question, I could not make out what the question is asking; i.e. I did not understand (yhe bold part) that a is combined with b to form c. Re: A chemist combined 'a' milliliters of a solution that   [#permalink] 11 Nov 2018, 06:19
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