JAMB 2014 · UTME · Q19

The 4th term of an A.P. is 13 while the 10th term is 31. Find the 24th term.

Worked solution (try it first)

From the 4th term to the 10th is 6 steps of $d$ : $6d = 31 - 13 = 18$ , so $d = 3$ .
Go back three steps from the 4th term: $a = 13 - 3 \times 3 = 4$ .
The 24th term is $a + 23d = 4 + 69 = 73$ , option D.