Question 776837
Let the 1st term be 'a' and the common difference be 'b'.
Therefore the 5th term is a+4b = 10(given)----------------(1)
The formula for the sum of 15 terms of an A.P. is
     S(10)= (10/2)(2a+9b)=115 
  or S(10)= 5(2a+9b)=115 
  or S(10)= 10a+45b =115 ----------------------------------(2)
  
Multiplying equation1 by 10 we have 10a+40b = 100 --------(3)
Subtracting equation3 from equation2 we have 5b= 15
So we have b=3(common difference)

Putting the value of 'b' in equation1 we have a+4*3=10
So a=10-12= -2(first term of the series)