I would round 23.25 up to 24, then double that to get 48, and finally move the decimal point 1 spot to the left to get 4.80
Note how 20% of 24 = 20/100*24 = 0.2*24 = 4.80
This is an overestimate of the actual tip which is...
20% of 23.25 = 0.2*23.25 = 4.65
The overestimate occurs because I rounded 23.25 up to 24. I rounded up because it is easier to deal with whole numbers. Also, you should round up (not down) to make sure you clear over the target hurdle.
