There is a connection or pattern in doing the mind reading, for example....
5 and ? = 45
You need to guess what is the number for ?. Let's look at the working.
a) 59 - 14 (5+9) = 45
b) 54 - 9 (5+4) = 45
This shows that as long as there is the equation sum falls under 9, there is no mistake to say that your answer will be right. Remarkable isn't it?