Marcus and tony work for lombardo's pipe and concrete. mr. lombardo is preparing an estimate for a customer. he knows that marcus lays a slab of concrete in 6 hours. tony lays the same size slab in 4 hours. if both work on the job and the cost of labor is $ 43.00 per hour, decide what the labor estimate should be.

Remark
The equation is job1/ time + job2/time = job3/time 

Discussion
job one is the time it takes one of the workers to complete the job.
job two is the time it  takes the other workers to complete the job
job three is the time it takes to do the job together. 

Givens.
Let Marcus's time = m
Let Tony's time be =t
Let their together time = t1

Equation Substitution and solution.
1/m + 1/t = 1/t1
1/6 + 1/4 = 1/t1  The common denominator on the left is 12.
2/12 + 3/12 = 1 / t1
5/12 = 1/t1          Cross multiply.
5*t1 = 12             Divide by 5
t1 = 12/5
t1 = 2 2/5
t1 = 2.4 hours

Cost
1 hour = 43 dollars for labor.
2.4 hours = x dollars for labor.

1*x = 43 * 2.4
x = 103.2 dollars. <<<<< Answer