The bar graph shows the number of files sold from March to July. In which of the following months was the percentage change in the number of files sold the greatest?

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.