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

Let's call Bricklayer Brenda as BB and bricklayer Brandon as BBN

BB completes the work in 9 hours
BBN completes the work in 10 hours

Let us assume the total work to be done as 900W( lcm of9,10)*10 ( Multiplied by 10 as there were other complications involved, so maybe a larger number will simplify calculation

Work per hour done by BB = 900W/9 = 100 W/h
Work per hour done by BBN = 900W/10= 90 W/h

Combined work if both did not waste time and work efficiently= 190 W/h

But we know that total work was completed in 5 hours, Hence, the actual work done per hour by BB+ BBN= 900W/5 = 180 W

The difference between actual work and work expected from BB=BBN = 190W - 180W = 10W

We know their combined output give 10 bricks less per hour.
This means 10W= 10 bricks

W= 1 brick ....(1)

Total work for chimney construction = 900W = 900 bricks (Subtituting from (1) above)
Correct Answer (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)
_________________

rate of Brenda = 1/9
and Brandon= 1/10
together = 1/9+1/10; 19/90
they can build together at 1/5 hrs ; 19/90-1/5 ; 1/90 so rate = 90 and time = 10 hrs
work done = 90*10 ; 900
IMO B

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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