Question

How many terms of the AP 3,5,7,9... must be added to get the sum 120 ?

Answer 1

a=3;
d=5-3=2
Sn=120
put formula of Sn
you'll get n

Answer 2

the answer is n=47 by keeping a=3,d=5 and Sn=120 ,applying these in the formula Sn=n/2(2a+(n-1)d) you will get n=47