The 4th term of an A.P. is 13 while the 10th term is 31. Find the 21st term.
- 175
- 85
-
64
- 45
The correct answer is: C
Explanation
a + 3d = 13a + 9d = 31
6d = 18
= d = 3
a = 13 - 9
= 4
a + 20d = 4 + (20 x 3)
= 64