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

Problem: a man is 28 years older than his son . in two years , his age will be twice the age of his son . the present age of the son is ?
26

Problem: income and expenditure of a person are in the ratio 7 : 6 . if the income of the person is rs . 14000 , then find his savings ?
2000

Problem: an army ' s recruitment process included n rounds of selection tasks . for the first a rounds , the rejection percentage was 60 percent per round . for the next b rounds , the rejection percentage was 50 percent per round and for the remaining rounds , the selection percentage was 70 percent per round . if there were 10000 people who applied for the army and 350 were finally selected , what was the value of n ?
9