I agree with Mystery, find the average of the digits, then add on 40.
There is a similar question which often comes up about finding a missing number if you are given the mean. So say you were given the mean of 45, but not the last number of 41. The text book way to find the number is to add all the numbers then subtract this from 45*10.
The tip I gave dd was to go through the numbers giving them a value of how much less or more they are from 45. So for the list given the numbers you would get are
This adds up to 4. So the missing number must be 4 less than 45.