Since these seem to be logic problems rather than straight maths, 20 is too obvious an answer.
It takes ONE brick to complete the wall. After the first 19 it is still incomplete!
Thanks guys very helpful. Here are some more riddles (lateral thinking required)
1. What kind of running means walking?
2. If there are 3 apples and you take away 2 how many do you have?
3. Why is 7 x 7 = 50 like your left eye?
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