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In New York City's funny math, you get only one teacher for the price of two.

The Department of Education pays about 1,500 teachers for time they spend on union activities -- and pays other teachers to replace them in the classroom.

It's a sweetheart deal that costs taxpayers an extra $9 million a year to pay fill-ins for instructors who are sprung -- at full pay -- to carry out responsibilities for the United Federation of Teachers.

With Mayor Bloomberg calling for thousands of teacher layoffs to balance the 2012 budget, critics say it's time to halt the extravagant benefit.

"In these tight fiscal times, it defies common sense to pay two different people to do one job," said Dick Dadey, executive director of Citizens Union, a government watchdog. "It's a waste of money."

That $9 million would cover the salaries of 198 new teachers at the current annual $45,530 starting pay.

The DOE lets 40 experienced teachers collect top pay and fringe benefits, but work just one class period a day.

Under a contract agreement since 2003, the DOE excuses these veterans to work for the UFT -- currently 38 as district representatives and two as union vice presidents. The UFT pays them another salary, plus expenses.

English teacher Tom Dromgoole, for instance, collects top teacher pay, $100,049 a year, from the DOE for his slot at Leadership and Public Service HS in downtown Manhattan. But he is relieved for most of the day to serve as a UFT high school rep. The UFT supplements his salary by $50,461, records show.

Click for more on this story about teachers being paid to do union business from the New York Post.