Profit percentage of September : $\dfrac{Sales-Cost}{Cost} \% = \dfrac{39-37}{37}\% = \dfrac{2}{37}\% = 5.40\%$
Profit percentage of July : $\dfrac{Sales-Cost}{Cost} \% = \dfrac{36-35}{35}\% = \dfrac{1}{35}\% = 2.85\%$
Profit percentage of March : $\dfrac{Sales-Cost}{Cost} \% = \dfrac{35-30}{30}\% = \dfrac{5}{30}\% = 16.66\%$
Profit percentage of May : $\dfrac{Sales-Cost}{Cost} \% = \dfrac{36-34}{34}\% = \dfrac{2}{34}\% = 5.88\%$
∴ As we can see March records the highest profit.