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Given: 15 liters are taken of from a container full of liquid A and replaced with Liquid B. Again 15 more liters of the mixture is taken and replaced with liquid B.
Asked: After this process, if the container contains Liquid A and B in the ratio 9:16, what is the capacity of the container?

Let the capacity of the container be V

First Replacement
Liquid A = V-15
Liquid B = 15
Total = V
Liquid A/Total = (V-15)/V
Liquid B/Total = 15/V

Second replacement
Liquid A = V-15 - 15(V-15)/V = (V-15)(1-15/V) = (V-15)ˆ2/V
Liquid B = V - (V-15)ˆ2/V = {Vˆ2 - (V-15)ˆ2}/V
Ratio Liquid A and Liquid B = (V-15)ˆ2 / {Vˆ2 - (V-15)ˆ2} = 9/16
(V-15)ˆ2/Vˆ2 = 9/25
(V-15)/V = 3/5
V/15 = 5/2
V = 75/2 = 37.5

IMO C
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­-> Inital A:B = x:0
Operation 1
-> 15L removed. A:B = x-15:0
-> 15L of B added. A:B = x-15:15
Proportion of A in the mix: \(\frac{x-15}{x}\)
 
Operation 2
-> 15L of mixture removed. 
Amount of A removed when 15L of mixture is removed = \(\frac{x-15}{x}\) x 15­
Thus,
Amount of A in the mixture after the removal = (x-15) - \(\frac{x-15}{x}\) x 15­ = \((x-15)^{2}\)/x­

-> 15 L of B is added. Final A:B = 9:16.

Final amount of A in the mixture = \((x-15)^{2}\)/x­ = 9/25 into final total volume

Final total volume is still x (15 l is removed, 15 l is added in each operation).

So,
\((x-15)^{2}\)/x­ = 9/25 (x)
\((x-15)^{2}\)/\(x^{2}\)­ = 9/25
=> \(\frac{x-15}{x}\) = 3/5
=> 5x - 75 = 3x
=> 2x = 75
=> x = 75/2 = 37.5 = capcacity of container. Choice C.

Harsha
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