All we need to do is to divide August and September rainfall measurements into two equal halves and then substract 0.6 from one of them and add 0.6 to the other one.
6.2 ÷ 2 = 3.1
3.1 - 0.6 = 2.5
3.1 + 0.6 = 3.7
It's said that in August there was 0.6 inch more rainfall, so the bigger value (3.7 inches) is for August and the smaller value (2.5 inches) is for September.
