Brenda can do 1/9th of the work in 1 hr and Brandon can do 1/10th of the work in 1 hr.
Working together they can do (1/9+1/10)=19/90th of the work in 1 hr which means that together they can complete the work in 90/19 hours.
Now let the total number of bricks in the chimney be 'x'.
So had they worked at their usual speed (without wasting time by gossiping) they would would have laid 'x/(90/19)'=19x/90 bricks in 1 hr. But they did gossip and ended up finishing the work in 5 hours which means that they laid x/5 bricks in an hour.
We are told that their combined output decreased by 10 bricks/hr due to the time they wasted.

Thus, 19x/90 - x/5 = 10. Solving we have x=900. Ans.: B

Rate of work of Brenda = 1/9
Rate of work of Brandon = 1/10
Combined rate should be 1/9 + 1/10 = 19/90 but it is actually 1/5

Decrease in rate = 19/90 - 1/5 = 1/90th of the chimney per hour.

But we are given that the decrease in rate is actually 10 bricks per hour so 10 bricks = 1/90th of chimney
No of bricks in the chimney = 10*90 = 900 bricks

Answer (B)
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Let total work be divided into 90 parts.
Brenda completes 10 parts per hour.
Brandon completes 9 parts per hour.
In ideal scenario they should have completed 19 parts together in 1 hour but they take 5 hours to complete the work together which comes down to 18 parts per hour.
This reduction in 1 part per hour is due to the reduction in 10 bricks per hour (as stated in the question)
1 part = 10 bricks.
90 parts (total work) = 10*90 = 900 bricks.

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