The 4th term of an A.P. is 13 while the 10th term is 31. Find the 21st term.
A
175
B
85
C
64
D
45
Verified Correct Answer: C.
Step-by-Step Mathematical Solution
a + 3d = 13
a + 9d = 31
6d = 18
= d = 3
a = 13 - 9
= 4
a + 20d = 4 + (20 x 3)
= 64
Source: Myschool Past Questions Archive (Record #30910)