This reminds me of a story.
When my son was in 2nd grade, my mom took him to a garage sell. My son had about $5 to buy toys. My son picked out of few things to buy, and then he told the person at the cash register what the change would be.
The person rang up the purchase on the cash register (or calculator) and my son was correct. My son then figured out some more change for people, and was amazing all the adults around.
I think my mom was quite proud of him.