$\begin{array}{ccccccc}Name& Month1& Month2& Month4& Month4& Month5& Average\\ Suresh& 12& 27& 37& 47& 57& 36\\ Virat& 11& 21& 31& 46& 56& 33\\ Rohit& 9& 24& 34& 44& 54& 33\\ Ajinkya& 10& 20& 35& 45& 55& 33\\ Mahendra& 15& 25& 35& 50& 60& 37\\ Shikar& 21& 31& 46& 56& 66& 44\end{array}$
Had each of the six planted 10 trees more in a month than in the previous month for all the five months, their averages would be:
Suresh: 32, Virat: 31, Rohit: 29, Ajinkya: 30, Mahendra: 35, Shikhar: 41.
But the actual averages are: Suresh: 36, Virat: 33, Rohit: 33, Ajinkya: 33, Mahendra: 37, Shikhar: 44.
Since there is a difference of 4 between the expected average and actual average, Suresh planted 4 × 5 = 20 more trees than expected. Similarly, Virat, Rohit, Ajinkya, Mahendra and Shikhar planted 10, 20, 15, 10 and 15 more trees respectively than expected.
Now If any of these people had planted 15 trees more than the previous month in month 5, then the number of excess trees planted would only have been 5. Similarly, had the person planted 15 trees more than the previous month in month 4, the number of excess trees planted would be 5 + 5 = 10. Thus, if a person planted 15 trees more than the previous month in month 2, month 3, month 4 or month 5, the corresponding number of total excess trees would be 20, 15, 10 and 5 respectively. Thus, Suresh, Virat, Rohit, Ajinkya, Mahendra and Shikhar planted 15 more trees than the previous month in month 2, month 4, month 2, month 3, month 4 and month 3 respectively.
Thus, the number of trees planted by them in each month can be calculated as shown in the table above.
Thus, Suresh planted 47 trees in month 4.
Alternatively,
From month 2 onwards, each person plants 10 more trees in every subsequent month and 15 more trees than the previous month in exactly one month. If the person plants x trees in month 1 and had planted exactly 10 more trees in each subsequent month, the total number of trees planted by that person would be x + (x+ 10) + (x+ 20) + (x+ 30) + (x+ 40) = 5x+ 100.
Now, if the person had planted 15 trees more than the previous month in month 2, the total number of trees would be x + (x+ 15) + (x+ 25) + (x+ 35) + (x+ 45) = 5x+ 100 + (5 × 4). On the other hand, if the person had planted 15 trees more than the previous month in month 5, the total number of trees would be x+ (x+ 10) + (x+ 20) + (x+ 30) + (x+ 45) = 5x+ 100 + (5 × 1). Thus, if a person had planted 15 more trees than the previous month in month, the total number of trees planted by that person = 5x+ 100 + (5 × No. of months to go from month of planting to end of tenure (p)).
Total number of trees planted by Suresh = 36 × 5 = 180
∴(5 × 12) + 100 + 5p= 180 ⇒p= 4
This simply means that Suresh planted 15 more trees than the previous month in month 4.
Using this approach, Virat, Rohit, Ajinkya, Mahendra and Shikhar planted 15 more trees than the previous month in month 4, month 2, month 3, month 4 and month 3 respectively.

Shikhar planted 15 more trees than in the previous month in month 3.