The first bricklayer can build a wall in 15 days, the second bricklayer can build the same wall in 20 days. How long will it take them to build the wall together?

It takes 8  4/7 days for both bricklayers to build the wall together.

Step-by-step explanation:  

The first bricklayer takes 15 days to build the wall, which means it builds 1/15 of the wall per day.

The second bricklayer takes 20 days to build the wall, which means it builds 1/20 of the wall per day.

So if they build the wall together, the expression below will tell how much of the wall they build per day together.

1/15 + 1/20

To add the fractions, we must find a common denominator, which is 60.  

4/60 + 3/60, which equals 7/60.

So, they can build 7/60 of the wall per day together.  How many days will it take them?  We must find how many times 7 fits into 60, which can be done by solving 60/7, which is 8 4/7.  

So, it takes 8 4/7 days for both bricklayers to build the wall together.

step-by-step explanation: