y 与 $\sqrt[3]{x}$ 成正比。
当 x 增加 700% 时，求 y 增加的百分比。 答 ... % 6 个人粉刷一面壁画墙需要 50 小时。
4 个人粉刷同一面墙需要多长时间？ 答 ... 小时

1. Convert the percentage increase in $x$ into a multiplication factor.

An increase of $700\%$ means the new value of $x$ is
$100\%+700\%=800\%$
of the original value.

So
$x_{\text{new}}=8x_{\text{old}}.$

2. Use the direct proportion relationship to find the percentage increase in $y$.

Since
$y \propto \sqrt[3]{x},$
when $x$ is multiplied by $8$,
$y \text{ is multiplied by } \sqrt[3]{8}=2.$

So the new value of $y$ is $200\%$ of the original value.

Percentage increase in $y$:
$200\%-100\%=100\%.$

1. Find the total work in man-hours.

For the same mural wall,
$6 \times 50 = 300$
So the work required is $300$ man-hours.

2. Divide the total man-hours by the new number of men.

Time taken by $4$ men:
$300 \div 4 = 75$