Question

Bricklayer Brenda would take $9$ hours to build a chimney alone, and bricklayer Brandon would take $10$ hours to build it alone. When they work together, they talk a lot, and their combined output is decreased by $10$ bricks per hour. Working together, they build the chimney in $5$ hours. How many bricks are in the chimney?

$500$

$900$

$950$

$1000$

$1900$

Answer: B

Concepts:rate linear equation

Difficulty rating: 1530

Solution:

Let $n$ be the number of bricks. Alone, Brenda lays $\tfrac{n}{9}$ bricks per hour and Brandon lays $\tfrac{n}{10}.$ Together, their rate is $\tfrac{n}{9} + \tfrac{n}{10} - 10.$

Working for $5$ hours completes the chimney: $5\left(\tfrac{n}{9} + \tfrac{n}{10} - 10\right) = n.$ Expanding, $\tfrac{5n}{9} + \tfrac{5n}{10} - 50 = n,$ so $\tfrac{95n}{90} - n = 50,$ giving $\tfrac{5n}{90} = 50.$

Hence $n = 900.$

Thus, the correct answer is B.

2008 AMC 12B Problem 10

Answer: B

Difficulty rating: 1530

Solution:

Problem 10 in Other Years