Some people have the ability to add numbers quickly in their head. I have marveled at how some people need calculators for the simplest of math items when I have already figured it out in my head.
It s a sad commentary when cashiers need a machine to tell them how much change to give a customer.
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"It doesn't cost "nuttin", to be nice". 
MOM
I just want to do the right thing! Uncle Joe, (my hero).