A and B can build a wall in 15 and 20 hrs resp.
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.
if they worked together they use 180 bricks less
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.
and build the wall in 12 hrs .
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,
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Edwin
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