该条形图显示了从3月到7月售出的文件数量。 在以下哪个月份中，售出文件数量的百分比变化最大？

1. Read the number of files sold for each month from the bar graph

March: $40$
April: $50$
May: $40$
June: $20$
July: $8$

2. Calculate the percentage change from March to April

Change = $50 - 40 = 10$
Percentage change = $\frac{10}{40} \times 100\% = 25\%$

3. Calculate the percentage change from April to May

Change = $50 - 40 = 10$
Percentage change = $\frac{10}{50} \times 100\% = 20\%$

4. Calculate the percentage change from May to June

Change = $40 - 20 = 20$
Percentage change = $\frac{20}{40} \times 100\% = 50\%$

5. Calculate the percentage change from June to July

Change = $20 - 8 = 12$
Percentage change = $\frac{12}{20} \times 100\% = 60\%$

6. Compare the percentage changes to find the greatest

The percentage changes are $25\%$, $20\%$, $50\%$, and $60\%$.
The greatest percentage change is $60\%$, which occurred from June to July.