Solutions
The number of candidates applied for post P in 2015 = 342
The number of candidates applied for post Q in 2012 = 320
The number of candidates applied for post Q in 2013 = 375
The number of candidates applied for post Q in 2014 = 424
The number of candidates applied for post Q in 2015 = 452
The number of candidates applied for post Q in 2016 = 284
The number of candidates applied for post Q in 2017 =455
⇒ Total number of candidates applied for post Q = 320 + 375+ 424 + 452 + 284 + 455 = 2310
Now, required percentage =

15%
Hence, the number of candidates applied for post P in 2015 is approx. 15% of the total number of candidates applied for post Q.