Question number 37 olympiad 2013 section B

Solution :
Income on the nth day is =n^2+2..........(1)
Expenditure on nth day=2n+1..............(2)
Therefore ,the saving on nth day=n^-2n+1.....(3)
The savings on day1 to nth day when summed  up =1240......(Given)
The saving series from day-1  to nth day as follows like 0,1,4,9,16,----------n^2
Sum of this type of series= n(n+1)(2n+1)/6....................................Not A.P or G.P
Here the value of n=n-1 as first day saving is 0
Threfore ,(n-1)(n)(2n-2+1)/6=1240,=>n=16(Answer)