Market math

Fiona watches Sophie adeptly add the cost of three items in her head, receive a $20 bill, work out the required change and count out a $5 bill, two toonies and three quarters to make change quickly and accurately.

"Sophie's not very good at making change," she whispers to me quietly.

"What do you mean?" I ask, stupidly assuming that Fiona was unable to follow the complexity of what Sophie has just done and thinks she made a mistake.

"She should have just asked that lady if she had an extra quarter and given her a ten back. Much simpler."