Quantitative Aptitude – Arithmetic – Mixtures – A 20% ethanol solution is mixed

Slot -2 – Quantitative Aptitude – Arithmetic – Mixtures – A 20% ethanol solution is mixed

A 20% ethanol solution is mixed with another ethanol solution, say, S of unknown concentration in the proportion 1:3 by volume. This mixture is then mixed with an equal volume of 20% ethanol solution. If the resultant mixture is a 31.25% ethanol solution, then the unknown concentration of S is?

a) 55%
b) 52%
c) 48%
d) 50%

Answer: d) 50 %

Solution:
Let the volume of 20% ethanol solution in first mixtures = x so
Volume of S = 3x
Total volume of first solution = 4x
Total volume of final solution =2×4x=8x
Volume of ethanol solution in final mixture = x + 4x = 5x ,
So volume of S in final solution = 3x
Thus (31.25-20)/(s-31.25) = 3x/5x = 3/5
56.25=3s-93.75
3s=150
s=50%

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