How quickly can you spot the missing number?

Most of us will be fairly confident that we can count from one to 50 without any struggle, however this puzzle may just have you questioning your numeracy skills.

This puzzle does seem fairly straightforward at a glance, but the missing number can be nearly impossible to spot.

Can you tell which number is missing from this puzzle on Diply? You can check out the gallery above for the answer. 

Seems like an easy task, but to be frank, you actually had to look twice.