3 : 12 :: 5 :?

First number = 8 and the sum of the digits of the second number is 2 + 8 = 10.

Thus the difference of the first number and the sum of the digits of second number is 10 - 8 = 2.

Similarly, the sum of the digits of third number is 2 + 7 = 9.

Hence the sum of digits of fourth number should be 2 more than 9 i.e. 11

Hence, fourth number is 65.

As, 123 ? 13²

As, 235 ? 25³

The middle digit of first term becomes power to the next term.

14 = (2 x 9 - 4)

26 = (2 x 15 - 4)

? = 15

As position of M and N in Eq. alphabets are 13 and 14 respectively.

As, 16:56 = (2/7)

Similarly, 32:112:(2/7)

As, (4)² + 3 = 19

Similarly, (7)² + 3 = 52

As 24:60 = (2/5)

Similarly, (120/300) = (2/5)

As 335 - 216 = 119

Similarly, 987 - X = 119

Therefore, X = 987 - 119 = 868.