UPSEE 2017 Solved Paper

We consider two possible cases about tuning forks A,B and C.
Case I When A and B are tuned, four beats are produced. When B and C are tuned, five beats are produced.
So, there is a difference of 1 .|nA−nC|=1
Case II When A and C are tuned, the total number of beat produced.|nC−nA|=4+5=9