Answer:
6 hours
Explanation:
The trick to this type of problem is using an equal amount of time.
The least common multiple of 7 and 42 is 42
At 7 hours per wall
In 42 hours the bricklayer could build
42/7 = 6 walls
At 42 hours per wall
In 42 hours the apprentice could build
42/1 = 1 walls
Working together in 42 hours they could build 6 + 1 = 7 walls
42hours/7walls = 6hours/ wall
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Math only
x = 1 wall/7hours + 1wall/42 hours
use common denominator of 42
x = 6w/42 hr + 1w/42hr
x = 7w/42 hr
reduce
x = 1w / 6hr