Presently a mixture in a tub contains substances A and B in the ratio of 7:5. If 9 litres of this solution is drawn out and B is added to the mixture in certain quantity the resulting mixture becomes 7:9. How much litres of liquid A was kept in the tub initially?
Answer: D
Currently the mixture is in ratio of A/B: 7:5
=> This means fraction of A in the solution: 7/12
=> Fraction of B in the solvent: 5/12
=>New concentration of A= 7-(7/12)9
=>New concentration of B= 5-(5/12)9+x
=> (7-(7/12)9)/(5-(5/12)9+x) = 7/9
=>x=21