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Question:

Which term of the AP 3, 15, 27, 39,... will be 120 more than its 21st term?

Solution:

A.P=3,15,27,39,..From the above A.P , we know thata=3andd=12A21=a+20d=3+240=243Lets suppose the term of AP will be 120 more than its21stterm beAnAn=A21+120=243+120=363An=a+(nš•’µ)d363=3+(nš•’µ)12363š•’·=(nš•’µ)1236012=nš•’µnš•’µ=30n=31so,31stterm of the AP will be120more than its21stterm.