Question

Отец и сын, работая вместе, покрасили забор за 12 ч. Если бы отец красил забор один, он выполнил бы работу за 21 ч. За сколько часов покрасил бы этот забор сын? Ответ должен быть 28 ч, нужно решение

Answer 1

1/12 совместная производительность,1/21-пр.отца
1/12-1/21=1/28 производительность сыны
1(весь забор):1/28=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 takes x hours to paint the fence alone.

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

1/21 + 1/x = 1/12

To solve this equation, we can find a common denominator and then simplify:

12x + 12(21) = 21x

Simplifying further:

12x + 252 = 21x

252 = 21x - 12x

252 = 9x

x = 252/9

x = 28

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

Answer

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