null hypothesis

A retail chain store knows that on average, sales in its stores are 20 percent higher in December than in November. For the current year, a random sample of six stores was selected. Their percentage December sales increases were found to be
19.2
18.4
19.8
20.2
20.4
19
Assuming a normal population distribution, test the null hypothesis that the true mean percentage sales increase is 20, against the two-sided alternative, at the 10% significant level.