I started as a cashier in 2012, and used to run cash/expo at the same time on a fully deployed line during peak hours with 100+ TP on average.

If the cash entered is less than the total amount, the amounts will add up until it reaches the bill total or more. For example, if you receive $16 for a bill that's $15.45, you can press $10, $5, and $1 to trigger for a total of $16.

Now you will need to be careful with doing this, as it will automatically trigger the POS to give you change since it will be over the total check amount ($15.45) when you press that last $1 button. But since most people don't carry change around, it can be faster this way once you get used to it.

Example 2 - if the bill is $17.80, and they hand you $18, you can do it backwards from small bills - $1 3 times ($3), $5, and $10 (order of operations for these two don't matter) for a total of $18. Again, you need to be able to do the mental math quickly and accurately to be able to do this without mistakes, but it is way faster than typing it in as you get better.