In a LAB experiment , a 100 ml compound of element X

[gmatquant25]

In a LAB experiment , a 100 ml compound of element X and element Y needs to be increased to 120 ml by adding some quantity of element X, and some quantity of element Y .If the original compound contains 30% of element X , how much (in ml) of element Y must be added so that element X will be exactly one third of the resulting mixture ?


A)40 ml
B)100/3 ml
c)20 ml
D)20/3 ml
E)10 ml

CAN this be solved using allegation method ?

OA E

[vivekchandrams]

Hi gmatquant25,

I dunno which method you are referring to, but here's the one which I typically use.

Original mixture contains 30 ml of X and 70 ml of Y.

Now the new volume of solution is 120 ml of which 30% has to be X.

So the volume of X in the new solution is 30*120/100 = 40 ml.

So, in the new mixture of 120 ml, vol of X is 40 ml. So the remaining 80 ml has to be of Y.

Hence, the volume of Y to be added is 80 - 70 = 10 ml

Hope it helps.


Pls hit the 'thank' icon if you find my post useful

[Atekihcan]

Now the new volume of solution is 120 ml of which 30% has to be X.

So the volume of X in the new solution is 30*120/100 = 40 ml.

30*120/100 = 36 not 40

Your mistake is in the final solution 1/3 of 120 ml has to be X NOT 30%.

Hope that helps.

[vivekchandrams]

I'm sorry. I'm really sorry. I wanted to mean that but just swayed away while typing. Thanks for letting me know

[GMATGuruNY]

In a LAB experiment , a 100 ml compound of element X and element Y needs to be increased to 120 ml by adding some quantity of element X, and some quantity of element Y .If the original compound contains 30% of element X , how much (in ml) of element Y must be added so that element X will be exactly one third of the resulting mixture ?


A)40 ml
B)100/3 ml
c)20 ml
D)20/3 ml
E)10 ml

OA E

Amount of X in the original 100ml solution = .3(100) = 30.
Amount of X in the final 120ml solution = (1/3)(120) = 40.
Increase in X = 40-30 = 10.
Total increase in volume between the two solutions = 120-100 = 20.
Since the total increase in volume is 20ml, and the increase in X is only 10ml, the amount of Y added = 20-10 = 10.

The correct answer is E.