Question

which term of the AP 3 15 27 36....will be 120 more than its 21st term

Answer 1

let term =an
then
an=120+a+20d
an=120+3+240
an=. 363
3+(n-1)12=363
(n-1)12=360
n-1=30
n=31
Hence term is 31.
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Answer 2

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