Question

Aamir would take 9 hours to build a chimney alone, and Saif 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?

• A. 500
• B. 900
• C. 950
• D. 1000
• E. 1900

Answer 1

The correct option is B

900

Using the conventional approach :

Saif takes 10 hrs.
So total time they both would take without talking $=\frac{10\times 9}{19}=\frac{90}{19}$ hrs.
But they take 5 hrs $=\frac{95}{19}$ hrs.

So time wasted is $\frac{5}{19}$ hrs.

They work for 5 hrs and put 10 bricks less per hour, so they had to put in extra time for $5\times 10=50$ bricks.
Now, extra time taken= $\frac{5}{19}$ hrs.
so, in $\frac{5}{19}$ hrs: 50 bricks
5 hrs: x
x=950 bricks.
But this includes the 50 bricks extra put in the time $\frac{5}{19}$ hrs. So total number of bricks is 900.

Using percentage approach:

Aamir takes 9 hours,$\Rightarrow$ he does $11.11\%$ of the work every hour

Saif takes 10 hours $\Rightarrow$ he does 10% of the work every hour

Together they finish $21.11\%$ of the work

Given that they take 5 hours to finish the work after chatting, which means they are actually finishing $\frac{100}{5}=20\%$ of the work

The remaining $1.1\%$ accounts for the 10 bricks decrease due to chatting

Thus, $100\%$ corresponds to 900. Answer is option (b)