Choice C: 2 litres

The initial mixture contains 10% water out of the total 70 litres.

So, the initial mixture has 7 litres water and 63 litres alcohol.

What is done? Only water is added and after that the concentration of water becomes 12.5%

Fractional equivalent of 12.5% = 1/8

Method 1: Let x litres of water be added to the mixture.

So, we will have (7 + x) litres of water and (70 + x) litres of mixture finally.

Because 12.5% or 1/8 of the total mixture is water, (7 + x)/(70 + x) = 1/8

8(7 + x) = 70 + x

56 + 8x = 70 + x

Or 7x = 14

So, x = 2.

Method 2: The quantity of alcohol remains unchanged at 63 litres in the initial and final mixture.

After adding water, the new mixture will have 1/8 water or 7/8th of the new volume will be alcohol.

If 7/8th of the new volume = 63, new volume = 63 * (8/7) = 9 * 8 = 72 litres.

Initial volume = 70 litres. New volume = 72 liters.

Water added = 2 litres.

_________________

An IIM C Alumnus - Class of '94

GMAT Tutor at Wizako GMAT Classes & Online Courses