AMC 10 2008b Problem 18

Bricklayer Brenda takes $9$ hours to build a chimney alone, and bricklayer Brandon takes $10$ hours to build it alone. When they work together, they talk a lot, and their combined output decreases by $10$ bricks per hour. Working together, they build the chimney in $5$ hours. How many bricks are in the chimney? $\mathrm{(A)}\ 500 \mathrm{(B)}\ 900 \mathrm{(C)}\ 950 \mathrm{(D)}\ 1000 \mathrm{(E)}\ 1900$