The task is to generate the correct answer for a given math problem.

[EX Q]: Problem: what is the greatest prime factor of 2 ^ 10 - 1 ?
[EX A]: 3

[EX Q]: Problem: the average of runs of a cricket player of 10 innings was 33 . how many runs must he make in his next innings so as to increase his average of runs by 4 ?
[EX A]: 77

[EX Q]: Problem: a can contains a mixture of liquids a and b is the ratio 7 : 5 . when 18 litres of mixture are drawn off and the can is filled with b , the ratio of a and b becomes 7 : 9 . how many liter of liquid a was contained by the can initially ?
[EX A]:
42