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A formerly Amish truck driver destroyed a historic bridge in Indiana because she flubbed the math on the weight of her big rig, police said.

Mary Lambright, 23, drove a 30-ton tractor-trailer full of bottled water over the Paoli Bridge — a delicate iron span built in 1880 — at around noon on Christmas Day, causing it to collapse, according to the Herald Times.

The driver told cops in the town of Paoli she knew the bridge had a 12,000-pound limit but that she forgot how many pounds are in a ton.

“She’s a very inexperienced driver,” Paoli Police Chief Randy Sanders said, adding the woman left Amish life only a year ago.