Question

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

Answer 1

1:12=1/12 часть забора покрасили отец с сыном за 1час
1:21 = 1/21 часть забора покрасил отец за 1 час
1/12-1/21= 1/28 часть забора будет покрашена сыном за 1час
1:1/28=28 часов покрасит забор сын забор, если будет красить один.
Ответ: за 28 часов покрасил бы этот забор сын.

Answer 2

Problem Analysis

The problem states 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 denote the time it takes for the son to paint the fence alone as S.

According to the given information, the combined work rate of the father and son is 12 hours, and the father's work rate alone is 21 hours.

We can use the formula: 1/Combined Work Rate = 1/Father's Work Rate + 1/Son's Work Rate

Calculation

Using the given information, we can calculate the time it would take the son to paint the fence alone.

1/12 = 1/21 + 1/S

1/S = 1/12 - 1/21 1/S = 7/84 1/S = 1/12

So, the son would paint the fence alone in 28 hours.

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

Final Answer

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