Question

Отец и сын,работая вместе,покрасили забор за 12ч.Если бы отец красил забор один,он выполнил бы работу за 21ч.За сколько часов покрасил бы этот забор сын?

Answer 1

Отец с сыном за час выполнили  1/12 работы. Сам отец за час выполнил 1/21 работы, поэтому сын за час выполняет  1/12 - 1/21 = 1/28 работы.

Следовательно, он сам покрасит забор за 28 часов.

 

Answer 2

Problem Analysis

We are given that a father and son painted a fence together in 12 hours. If the father had painted the fence alone, it would have taken him 21 hours. We need to determine how long it would take the son to paint the fence alone.

Solution

Let's assume that the son can paint the fence alone in x hours.

From the given information, we can set up the following equation based on the work rates of the father and son:

1/21 (father's work rate) + 1/x (son's work rate) = 1/12 (combined work rate)

To solve for x, we can multiply both sides of the equation by 21x:

x + 21 = 1.75x

Simplifying the equation:

0.75x = 21

Dividing both sides by 0.75:

x = 28

Therefore, it would take the son 28 hours to paint the fence alone.

Answer

The son would take 28 hours to paint the fence alone.

Explanation

When the father and son work together, they can paint the fence in 12 hours. If the father worked alone, it would take him 21 hours to complete the job. By solving the equation 1/21 + 1/x = 1/12, we find that the son would take 28 hours to paint the fence alone