Indeed a very tough MENSA question but here is the answer. It is 16ANOTHER SO-CALLED TOUGH MENSA QUESTION:
Complete the number series:
1, 2, 4, 7, 11,?, 22.
(Give the logic)
Count the items starting with number 2. Add the counting numbers to the previous number e.g. to the FIRST one in the series add the first counting number 1. So 1+1=2.