3 times the 10th term is equal to 5 times the 20th term. Find the 20th term.?
Given, 3a10 = 5a20
⇒ 3(a + 9d) = 5(a + 19d)
⇒ 3a + 27d = 5a + 95d
⇒ 3a –5a = 95d – 27d
⇒ –2a = 68d
⇒ a = –34d
a20 = a + 19d = –34d + 19d = –15d
As we have only one relation between a and d, so we cannot find the value of a20.