Gino 可以在 4 小时内粉刷 6 块篱笆板，而 Danish 可以在 5 小时内粉刷 7 块篱笆板。 Gino 和 Danish 合作粉刷总共 21 块篱笆板。 如果他们继续以相同的速率粉刷，他们需要多长时间才能粉刷完 21 块篱笆板？ 将你的答案以小时和分钟表示，精确到最接近的分钟。 答案 ... h ... min

1. Find Gino's and Danish's individual painting rates.

Gino's rate $= \dfrac{6}{4}=\dfrac{3}{2}$ panels per hour.

Danish's rate $= \dfrac{7}{5}$ panels per hour.

2. Find their combined rate and the time taken for 21 panels.

Combined rate
$=\dfrac{3}{2}+\dfrac{7}{5}$
$=\dfrac{15}{10}+\dfrac{14}{10}=\dfrac{29}{10}$
panels per hour.

Time taken
$=21\div \dfrac{29}{10}$
$=21\times \dfrac{10}{29}=\dfrac{210}{29}$
hours.

3. Convert the time to hours and minutes, correct to the nearest minute.

$\dfrac{210}{29}=7.241379\ldots \text{ hours}$

Fractional part:
$0.241379\ldots \times 60=14.4827\ldots \text{ minutes}$

Correct to the nearest minute, this is $14$ minutes.

So the time taken is $7$ h $14$ min to the nearest minute.