One I use for percentages (especially multiples of 10%) is to divide by 10 and then multiply by whatever number you need to get the right percentage.
I.e. 20% of 20 = 20/5=4 nice and easy. but 20% of 17.8 I would find too hard to just divide by 5.
So I do 17.8/10= 1.78 (10%) x2= 3.56.
Works for multiples of 5% as well. you just do the same thing to the 10% level below and then add half of the original figure.
I.e. 35% of 56= 5.6x3=16.8 (30%) + 2.8 (half of 10%)=19.6.