起初，三个女孩一共有 8.7 m 的丝带。她们各自使用了相同长度的丝带装饰教室。Ai Le 用了她丝带的 40%，Bee Huan 用了她丝带的 10%，Cally 用了她丝带的 50%。最后还剩下多少厘米的丝带？ 答：___

1. Convert the percentages to fractions and make the numerators equal to represent the same amount used.

Ai Le used $40\% = \frac{40}{100} = \frac{2}{5}$ of her ribbon.
Bee Huan used $10\% = \frac{10}{100} = \frac{1}{10} = \frac{2}{20}$ of her ribbon.
Cally used $50\% = \frac{50}{100} = \frac{1}{2} = \frac{2}{4}$ of her ribbon.
Since they used the same amount, we equate the numerators to $2$ units.

2. Find the total number of units at first and calculate the value of 1 unit in cm.

Total units at first $= 5 + 20 + 4 = 29$ units.
Total length $= 8.7 \text{ m} = 870 \text{ cm}$.
$29 \text{ units} = 870 \text{ cm}$
$1 \text{ unit} = 870 \div 29 = 30 \text{ cm}$

3. Calculate the total amount of ribbon left.

Units left for Ai Le $= 5 - 2 = 3$ units.
Units left for Bee Huan $= 20 - 2 = 18$ units.
Units left for Cally $= 4 - 2 = 2$ units.
Total units left $= 3 + 18 + 2 = 23$ units.
Length of ribbon left $= 23 \times 30 = 690 \text{ cm}$.