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NEW YORK (PIX11) — Refilling your MTA card is a pretty standard procedure for anyone traveling underground in New York City.

But for those who pay attention to the details, you probably noticed that annoying leftover balance that’s either a nickel or dime, or just a few cents shy of a ride. In other words, it’s useless.

According to Ben Wellington, a data guru and assistant professor at the Pratt Institute, that odd number is no accident.

“I’d be surprised if the MTA didn’t notice the remaining tens of millions of dollars [that’s] showing up,” he told PIX11 News. “To say that they haven’t noticed anything would be surprising.”

Wellington, who took to his Tumblr “I Quant NY” broke it all down using some “old school” math.

The culprit in all this, Wellington says, are the amounts the MTA so graciously suggest to customers when buying a MetroCard.

“What they do is give you a 5% bonus when you buy a MetroCard, so when you use the pre-set buttons that are present on the machine,” Wellington explains, “you automatically end up with unusable remainder balance.”

Regardless of how you do the math, the odd balance is inevitable. So what’s the easy fix? For starters, ignore the MTA’s suggested amounts and ride to the beat of your own drum, or amount.

Instead, Wellington advises to select the “other amounts” tab and plug in the magic numbers of $9.55, $19.05 and $38.10.

Selecting these amounts will use up the card’s entire balance every time.

PIX11 reached out to the MTA to see if there was a simple fix. According to a spokesperson, it’s not so simple.

“These machines do not hold an infinite amount of change and the denominations are suggested to insure there is ample change to accommodate customers who pay with cash,” a spokesperson said in a statement.

“That being said, we will certainly look at this as part of the process involved in rolling out the next scheduled fare increase slated for next year.”

In the meantime, memorizing these so-called magic numbers will surely pay dividends.