CHICAGO (CBS) — Water – it’s a necessity to live, but the system in Chicago to pay for it is a mess.

For nearly a  year, the CBS 2 Investigators have showed you the archaic, random, mistake-prone billing.

Now, Brad Edwards introduces us to a man out thousands of dollars, because the city’s computers can’t handle fourth-grade math.

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“They are taking advantage of people,” Robert Jablonski said.

The city’s Water Management Department has a long history of slapping people with wrong bills. This time, it’s because the agency was incapable of simple subtraction.

Jablonski is paying the price – more than $5,000 – for the math mistake. His problem began after his water meter broke.

“For quite a few months, we were getting estimated bills,” he said.

For example, he received bills for $122 and $254, normal enough; but he didn’t notice the readings were estimates, rather than actual meter readings, until a $5,606 surprise – thousands more than he’d ever paid.

Jablonski knew something was amiss.

“The math in the system doesn’t add up,” he said.

The previous reading on his statement (measured in thousands of gallons used) was 1,450, which was correct, but then the city listed his current reading at 803, the number his old meter froze at when it broke.

The problem was, the city subtracted the previous reading from the current reading, resulting in a negative number of -647. You can’t use a negative amount of water, so the city’s computer went cuckoo and made the number positive.

So Jablonski got billed for 647,000 gallons of water in one month, not the typical 16,000 or so.

The bill went from measuring in the hundreds of dollars to the thousands.

“We have to pay for this huge usage of water, which we didn’t use,” he said.

Jablonski showed the math to the water department. The response was just as bad as their arithmetic.

“The only help that I received was that the person was able to put us on a payment plan,” he said.

On top of telling him he was stuck with the bill, the Water Department also gave him a three-year deadline to pay it.

“It’s not okay. It’s not okay that we have to deal with it,” Jablonski said.

After CBS 2 contacted the city’s Water Management Department, they acknowledged there was an error with Jablonski’s bill. They said they were contacting him to find out if he wants a credit to his account, or a refund.