Answer:
The dentist should fail to reject the Null hypothesis
Explanation:
From the question we are told that
The sample size is n = 400
The sample mean is

The level of significance is 5% = 0.05
The Null hypothesis is

The Alternative hypothesis is

The p-value is
Looking at the given data we can see that the p-value is greater than the level of significance hence the dentist should fail to reject the Null hypothesis