How to find the mean of a data set with a missing value?

If you are given a mean and asked for one of the data values, you can still use the basic method for determining the mean, but work backwards. Generally we add up all the values and then divide by the number of values. In this case, working backwards, we multiply by the number of values (instead of dividing) and then subtract (instead of adding). You should be left with a data value from the set. 

The answer is 6 weeks