(a + 9d) + (a + 10d) + (a + 11d) = 129
3a + 30d = 129
a + 10d = 43 …………..(1)
(a + 18d) + (a + 19d) + (a + 20d) = 237
3a + 57d = 237
a + 19d = 79 ………………(2)
‘d’ value substituting in equation (1)
a + 10(4) = 43
a = 43 – 40 = 3
∴ a = 3
∴ Required A.P. is 3, 7, 11, 15, 19,…