Adding it all up

I see "345" in the cash registry—three digits flashed in green light on a dark background, like how they had appeared in old calculators in the nineties. I’m in a queue at Wendy’s, buying bacon mushroom melt with fries and Coke Zero—my long delayed lunch. I hand two one-hundred peso bills, and two twenty-peso bills to the nice lady behind the counter—confidently, as if had, in fact, added them correctly in my mind. “Sir, kulang pa po,” she tells me. I rectify my error, a realization that although I like mathematics, I do not like arithmetic.