In the month of March, Sales is = $35$, whereas, in Jan, Sales is = $30$
∴ Percentage increase = $\dfrac{35-30}{30}\% = \dfrac{5}{30}\% = 16.66\%$
In the month of September, Sales is = $39$, whereas, in July, Sales is = $36$
∴ Percentage increase = $\dfrac{39-36}{36}\% = \dfrac{3}{36}\% = 8.33\%$
In the month of July, Sales is = $36$, whereas, in May, Sales is = $36$
∴ Percentage increase = $0\%$
In the month of May, Sales is = $36$, whereas, in March, Sales is = $35$
∴ Percentage increase = $\dfrac{36-35}{35}\% = \dfrac{1}{35}\% = 2.85\%$
∴ In March, the percentage increases in Sales two months before, the highest (which is $16.66\%$)