Or you can round a bit to make it easier -
35% of 56:
Do 35% of 60, which is 3 and a half times 6, ie 21. Take a little bit off because 56 is a bit less than 60, call the answer 20.
Or to be slightly more scientific, 35% of 60 is 21.
56 is 4 less than 60, or about 6% less (two-thirds of 10%).
So my answer should also be two-thirds of 10% less, or 21 minus 1.4, which is 19.6.