having a feel for the numbers
600 times 0.3 140 times 80
Example A full railway ticket costs £ 24.90
If a student can claim a third off, how much do they pay?
Subtract 8.30 from 24.90 to get 16.60.
Use this method only if you are absolutely sure that you have the right answer. Otherwise you would obviously lose out on the chance of getting consolation marks for method.