An experienced bricklayer can construct a small wall in 4 hours. an apprentice can complete the job in 12 hours. find how long it takes if they work together.

Experienced worker takes 4hr to complete so in 1hr it would be 1/4 completed.
Apprentice worker takes 12hrs to complete, so in 1hr it would be 1/12 completed.
Team effort X hours and 1/X part of the job completed in an hour.

Set up:

1/4 + 1/12 = 1/x
Get the denominators the same to add. So multiply the left side by 3.
3/12 + 1/12 =1/x
Combine like terms.
4/12 = 1/x
Cross multiply.
4x = 12
Dived by 4 on both sides
X = 3 hrs if they work together.

The answer is 3.