cumulonimbus wrote:

subhashghosh wrote:

W/M = 3/7

W1/M1 = 1/9 W2/M2 = 2/3

So Q1/Q2 = (2/3 - 3/7)/(3/7 - 1/9)

= (14 - 9)/21/(27 - 7)/63 = 5/21 * 63/20 = 3/4

(1) is sufficient

(2)

For Solution 1

M = W + 80

M + W = 100

For Solution 2

M = W + 10

M + W = 50

So we can find the ratios of M:W in solutions and using above alligation technique find the required ratio.

Answer - D

Hi Karishma,

Why is the ratio of S1 to S2 not equal to 1/2, by using this method:

W1/M1 = 1/9 W2/M2 = 2/3

So Q1/Q2 = (2/3 - 3/7)/(3/7 - 1/9)

= (14 - 9)/21/(27 - 7)/63 = 5/21 * 63/20 = 3/4

Which one is correct?

Because you don't average out the ratio; you average out the concentration of any one component where the weights used will be volume. Understand that when you find the average of a quantity, it should make physical sense.

Say you know that milk:water = 1:9 in a 100 ml solution.

When you do 1/9 * 100 ml, what do you get? What is 11.11 ml? Nothing

What you have to do is 1/10 * 100 ml = 10 ml (amount of milk in the solution). 1/10 is the concentration of milk in the solution and you multiply that by the volume of solution.

So here, you have to work with any one component. Say we work with water.

Avg concentration of water = 3/10

Concentration of water in solution 1 = 1/10

Concentration of water in solution 2 = 2/5 = 4/10

w1/w2 = (4/10 - 3/10)/(3/10 - 1/10) = 1/2

P.S. - PM me the link when you want me to reply on a thread. I opened this post by chance. I may not have seen your question directed to me otherwise.

_________________

Karishma

Veritas Prep GMAT Instructor

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