Hi there!
In order to use mental math, we'll round up each value to the nearest whole number. Here's what we'd have: 10 + 9 + 11 + 9 + tax. Next, add the values together. We know that 9 and 11 equal 20, so then adding another 9 would give us 29, and adding a 10 would give us 39. To add tax, we can round up to 10%, which would give us 3 dollars and 90 cents, which can be rounded up to 4 dollars. Lastly, add 39 and 4 to give you a mental math answer of $43.
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!