You can multiply most 2-digit numbers by 9 in your head. The most efficient way is to multiply the number by ten and then subtract the number. For example:
45 x 9
First of all multiply 45 by 10 which is 450.
Then subtract 45 from 450 which is 405.
This maths worksheet shows how to do this and then gives some practice questions – but you do have to be pretty good at subtraction to use this method.