Take the two digit number 45
Square it (45*45) to make 2025
Split this 4 digit number in half to make 20 and 25.
Add them (20+25) to make 45.
Which is what you started with.

My office is on the 3rd floor but there is no elevator so every day I walk up the stairs 2 at a time (for more exercise) but I'm always annoyed that there is one step left over. So the next day I go up 3 steps at a time but again there is one step left over. By counting I realise that even if I did 4,5 or 6 steps at a time there would always be one left over. And no matter how many steps I do (apart from 1 at a time or all of them) there will always be some left over.
How many steps are there?