The founder of Mindyourdecisions.com, Presh Talwalker, challenged people if they really knew the logic, they could answer this:
If 6+4= 210, then ?+??=123
Spoiler alert - solution below:
For example, let's take 9 + 2 = 711. How do you get 711 from those numbers?
9 - 2 = 7
9 + 2 = 11
Write those two answers next to each other and you get 711
So ?+? = 123
If you split the number in two: the answers you need to get are 1 and 23
See which numbers will add together to make 23. You need to also be able to subtract one from another to give you 1.
12 - 11 = 1
12 + 11 = 23
Write them together and you get 123 so the answer is 12 + 11 = 123
-nzherald.co.nz