Suppose there are N bricks in the wall.

A can lay N bricks in 15 hours, so A's brick-laying rate is 
N bricks per 15 hours or N/15 bricks per hour.

B can lay N bricks in 20 hours, so B's brick-laying rate is 
N bricks per 20 hours or N/20 bricks per hour.

Whoa!  180 bricks less per hour than what? 180 bricks less per
hour than A's rate in bricks per hour? -- or 180 bricks less per hour
than B's rate in bricks per hour?  We can't continue until we know 
what to subtract 180 from.  And you didn't tell us.  

I'll try to go on anyway without that and see what happens:

Their combined brick-laying rate is (N/15 + N/20) bricks per
hour.   Getting an LCD of 60, their combined brick-laying rate works 
out to be (7N/60) bricks per hour. 

That says their combined brick-laying rate is N bricks laid in
12 hours or N/12 bricks per hour.

So we set those equal:

7N/60 = N/12

Cross multiplying

84N = 60N

Dividing both sides by N

84 = 60

Sorry. This problem is botched.  Look at the problem again,
and carefully type it out word-for-word (don't abbreviate and 
don't leave anything out) in the thank-you note form below and 
I'll get back to you by email.

Edwin