Since 10% = 0.10 = 1/10, finding 10% of a number is equivalent to finding 1/10 of a number. An example is 'find 10% of 420' and you can convert the problem to 'find 1/10 of 420' which is simply 42. Easy!
To find 20% of a number, simply find 10% of the number and double the answer. An example is 'find 20% of 2000'. To find 10% of 2000 you can simple drop off a zero, so 10% of 2000 = 200. Since 20% is twice as large as 10%, then 20% of 2000 = 400. Easy.
Find 10% of 280. Answer: 28 Easy!
Find 20% of 480. Since 10% of 480 = 48, then 20% is twice as large. 20% of 480 = 96 Easy!
Find 20% of 5200 Since 10% of 5200 = 520, then 20% is twice as large. 20% of 5200 = 1040 Easy!
For each problem in this problem set you will need to identify 10% or 20% of a number. Strive for being able to accomplish the problems with ease and then strive for speed. Good luck and enjoy the challenge!