The ratio, by volume, of soap to alcohol to water in a

intial soap to alcohol = 2/50
final soap to alcohol = 4/50 = 2/25

initial soap to water = 2/100
final soap to water = 2/200

in fianl solution soap = 2x, alcohol = 25x, and water = 200x

as per question 25x=100, x = 4

water = 200 * 4 = 800 option E

INITIAL-
S:A:W
2:50:100

DOUBLED AND HALVED
S:A:W
4:50:-
1:-:100 OR 4:-:400

so A/W=50/400
100/x=50/400
x=800

How you get 1/100=4/400?? Why you multiply by 4??

We have that s/a=4/50 (s corresponds to 4) and s/w=1/100 (s corresponds to 1). To get s/a/w ratio we need s to correspond to the same number in both ratio, so we multiply 1/100 by 4 to get s/w=4/100.

The first thing we want to do is set up the ratio of the original solution using variable multipliers.

soap : alcohol : water = 2x : 50x : 100x

We are given that the ratio of soap to alcohol is doubled. Our original ratio of soap to alcohol is:

soap/alcohol = 2x/50x = x/25x

(Notice that we only canceled the 2 and 50; we didn’t cancel the x because it’s our ratio multiplier.)

If we double the ratio we multiply the entire ratio by 2, so we have:

2(x/25x) = 2x/25x

So now our new ratio is:

soap/alcohol = 2x/25x

Next we are given that the ratio of soap to water is halved. Our original ratio of soap to water is:

soap/water = 2x/100x = x/50x

If we halve the ratio, we multiply the entire ratio by 1/2, so we have:

(1/2)(x/50x) = x/100x

So now our new ratio is:

soap/water = x/100x

Next we must notice that the amount of soap in the two new ratios is not the same value. In the first new ratio, soap = 2x and in the second new ratio, soap = x. Thus, we need to make these values equal before continuing to the answer. To do this, we multiply the second ratio by 2/2, so we have:

soap/water = (2/2)(x/100x) = 2x/200x

Now we can set up the ratio of the altered solution.

soap : alcohol : water = 2x : 25x : 200x

Lastly, we are given that the altered solution will contain 100 cubic centimeters of alcohol.

With this we can set up the following equation and determine x:

25x = 100

x = 4 (This means that the ratio multiplier is 4.)

Thus, the altered solution will contain (200)(4) = 800 cubic centimeters of water.

Answer is E.

I think this problem can be solved with accuracy by setting up a table. Please see the attached table. The table is self explanatory. Let know if an explanation needed.

Answer = 800

option E

S : A : W = 2 : 50 : 100

The solution will be altered so that the ratio of soap to alcohol is doubled
The ORIGINAL ratio is S : A = 2 : 50.
NEW ratio is S : A = 4 : 50

the ratio of soap to water is halved
The ORIGINAL ratio is S : W = 2 : 100
NEW ratio is S : W = 1 : 100


If the altered solution will contain 100 cubic centimeters of alcohol, how many cubic centimeters of water will it contain?
We need to COMBINE our two ratios.
Take S : W = 1 : 100 and create the EQUIVALENT ratio S : W = 4 : 400

We can now combine both ratios to get: S : A : W = 4 : 50 : 400
We want water = 100
So, take S : A : W = 4 : 50 : 400 and rewrite as S : A : W = 8 : 100 : 800

Answer: E

Cheers,
Brent

My method is as follows:-
Step 1 : Present ratio S:A:W = 2 : 50 : 100
Step 2 : Altered Ratio S:A = 4:50 and S:W= 1:100
Step 3 : To make S common multiply S:W x 4 = 4:400
Step 4: The new ratio S:A:W = 4 : 50 : 400
Step 5: Let amount of water be x. Therefore, 50/400=100/x ...which on solving gives x=800

If soap and alcohol is doubled, why it is not 4:100 instead of 4:50

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Initially i also use to get confused with halving or doubling of ratio. Best way to understand is that instead of considering ratio in form S:A use form S/A and than perform the intended operation. In case doubled, S/A*2 =4:100 ....Similarly when halving the ratio S/A*1/2

I hope it is clear now