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Method to finding out the missing value:

1) Count how many intervals there are between the two given values, including the second value.
2) Find the difference between the two values and divide that by the number of intervals.
3) Keep adding the result from the first value, until you reach the required missing value.

Eg:

_ _ _ _ _ 12 _ ? _ 60 _ _ _ _ _

In this example, there are 4 intervals between the 12 and 60. The difference between 12 and 60 is 48, so we divide 48 by 4. This gives us 12. The missing value is two intervals ahead of 12, so we must add 12 two times. The answer, therefore, would be 36!