do you want to do them in your head then its not that easy you will have to do lots of practice.
I generally can multiply a few numbers in my head but I look for a number near it.
like this to calculate
55*1.8
I see that the number 1.8 is 2 - 0.2
so a trick i would use is
55 * 1.8 = 2*55 - 0.2*55
= 110 - 11 = 99
for the 0.2 i just moved the decimal over one digit in my head
so i really only did one 2* and one subtraction
the same thing with a different number
75 * 1.8 = 150 - 15 = 135
if you want to try by calculating
thats one trick that works sometimes
or round to a number you know the answer for
say 11 * 11
well I know 11 * 10 thats 110
so I add 1 * 11 more that 110 + 11 = 121
you can do that with big numbers
7500 * 34
no real tricks i can use here
i see 30* 75 = 2250
this is because 3*75 = 225 time 10 more is 2250
then time 4*75 and add it
300 so we have
2250 + 300
thats 2550
because it was 7500 we add two 0's making the answer
255000
thats how i do some quick calculations in my head