3,8,13,.,253d=8−3=5.Writing this A.P. in reverse order253,248,243,…,13,8,5Now, for the new A.P., we havea=253d=248−2;53d=−5We want to find20th term, son=20By using nth termtn=a+(n−1;)dwe havea20=253+(20−1;)(−5)⇒120=253+(19)(−5)⇒a20=253−95=15820thterm from the last term=158.