Question

Which term of the AP 3,15,27,3.... will be 120 more than its 21st term?

Answer 1

a=3

d=15-3=12

an=120+a21

an=120+[3+(21-1)*12]

an=120+243=363

an=a+(n-1)d

363=3+(n-1)12

363-3/12=n-1

30+1=n

n=31

req. term= 31