The first worker, working alone, does the work in 28 days, and the second worker

The first worker, working alone, does the work in 28 days, and the second worker, working alone, does the same work in 21 days. How many days will both workers complete this work working together?

Let’s calculate what part of the work the first worker will do in one day:

1:28 = 1/28 part – of all the work will be done by the first worker in one day.

Let’s calculate what part of the work the second worker will do in one day:

1: 21 = 1/21 part – of all the work will be done by the second worker in one day.

Let’s calculate how much of the work both workers will complete in one day, working together:

1/28 + 1/21 = 4/84 + 3/84 = 7/84 = 1/12 part – both workers will complete the work in one day, working together.

Let’s calculate how many days it will take for both workers to complete the work:

1: 1/12 = 1 * 12/1 = 12 days – both workers will do all the work.

Answer: in twelve days.