2 men can build a wall in 15 and 20 hrs. respectively. But if they work together they use 280 less bricks per hour and build a wall?? in 12 hrs. Find the no. Of bricks in the wall

Dear Student,

(Please understand that they will use 280 bricks less, when working together.)

Let the total number of bricks = x
=> A will use x/15 bricks per hour 
And, B will use x/20 bricks per hour.

Now when A & B work together, 
no. of bricks per hour used by them = (x/15+x/20 – 280)
no. of hours of their work = 12

Hence, forming an equation, we get,
=> (x/15+x/20 – 280) * 12 = x.
=> x = 8400  (Answer)

Regards.