Weve all been there.
Finished a nice meal with friends, waiter comes by all smiley and plops the receipt down in front of you.
..
You think "Quick brain! Whats 25% of $284.12??!"
The waiter returns. You panic and give them a 100% tip.
Lets stop the messing about, take out the phone, open our app - beep boop, and voila! $71.03
Job done.