A couple of ways to do mental math is to break apart a number when multiplying for instance.............14 x 16 is the same as
6 x 14=84
10 x 16=160
Added together it 244
The hard part about mental math is not the math itself but the remembering of numbers.
Another quick trick is something like 59 x 59=?
Round 59 up to 60.
Take 60 x 60= 3600. Subtract 2 x 60=120. Add 1
Works like a charm. There is not enough room to explain it but it will impress people that you are able to do numbers like this in your head.