To Find: we need to find a number of terms in the given AP.

Given: The series is 11, 18, 25, 32, 39, …. 207

a₁=11, a₂= 18,d=18–11 = 7 and a_n=207

(Where a=a₁ is first term, a₂ is second term, a_n is nth term and d is common difference of given AP)

Formula Used: a_n = a + (n–1)d

a_n= 207 = a₁ + (n–1)(7)

207– 11 = (n–1)(7) [subtract 11 on both sides]

196 = (n–1)(7)

28 = (n–1) [Divide both side by 7]

n = 29 [add 1 on both sides]

So there are 29 terms in this AP.