# Find the missing number

What number should replace the ?

• For the answer to be correct you need the 21 to be 20 – Moti Mar 1 '15 at 3:42
• I expected such a comment. If 21 was 20 then this would be a different puzzle. – Daniil Agashiyev Mar 1 '15 at 3:49
• So, the answer is not 15? – Moti Mar 1 '15 at 4:40
• No, it's not 15. – Daniil Agashiyev Mar 1 '15 at 4:43
• This puzzle was made by Nob Yoshihagara! I'm not sure what the policy on putting this in the post is, and I didn't want to edit for something this trivial. – Deusovi Jan 10 '16 at 22:48

• Thanks @Len . Added more explanation to the spoiler. I just didn't want to spoil it for the curious ones that can't help hover over spoilers while they're just looking for a hint. :) – Donbhupi Mar 1 '15 at 5:11