8, 9, 8, 7, 10, 9, 6, 11, 10,?, 12

Clearly, 5 + 3 = 8, 8 + 3 = 11, 11 + 3 = 14, .....

So, the series is an A.P. in which a - 5 and d = 3.

Let 320 be the nth term of the series.

Then, 320 = 5 + (n - 1) x 3 or (n - 1) = 105 or n = 106.

The pattern is x 3.

So, missing term = 108 x 3 = 324.

The pattern is x 2 + 1, x 2 + 2, x 2 + 3,.....

So, missing term = 59 x 2 + 4 = 122.

The given series consists of squares of consecutive prime numbers

i.e. 2², 3², 5²,.....,

11², 13², 17², 19².

So, missing term = 7² = 49.

Each term at an even place in the series is the product of its two adjacent terms.

Thus, if the missing term be x, then we have : 7 x x = 56 or x = 56 ÷ 7 = 8.

Each number in the series is one more than twice the preceding number.

So, missing term = (15 x 2) + 1 = 31.

Each number in the series is 2 less than thrice the preceding number.

So, missing number = (10 x 3) - 2 = 28.

The pattern is + 4, + 8, + 12, + 16......

So, missing term = 41 + 20 = 61.

The pattern is - 66, - 55, - 44, - 33, - 22, .....

So, missing term = 105 - 11 = 94.

The pattern is + 3, + 5, + 7, + 9, .....

So, missing term = 28 + 11 = 39.