66, 36, 18, ?

Answer: Option C

Explanation:

Each term in the series is the product of the digits of the preceding term.

So, missing term = 1 x 8 = 8.

In the series 3, 9, 15, ...... what will be the 21st term?

Clearly, 3 + 6 = 9, 9 + 6 = 15,.....

So, the series is an A.P. in which a = 3 and d = 6.

Therefore 21st term = a + (21 - 1) d = a + 20d = 3 + 20 x 6 = 123.

28, 33, 31, 36, ?, 39

Answer: Option B

The pattern is + 5, - 2, + 5, - 2,.....

So, missing term = 36 - 2 = 34.

1, 9, 25, 49, 81, ?

The series consists of squares of consecutive odd numbers

i.e. 1^{2}, 3^{2}, 5^{2}, 7^{2}, 9^{2},.....

So, missing term = 11^{2} = 121.

1, 9, 25, 49, ?, 121

The given series consists of squares of consecutive odd numbers

i.e. 1^{2}, 3^{2}, 5^{2}, 7^{2},.....

So, missing term = 9^{2} = 81.