Answer:
There is enough not evident to support dealer's claim that his sales is no more than $80.
Step-by-step explanation:
We are given the following in the question:
Population mean, μ = $80
Sample mean,
= $91
Sample size, n = 20
Alpha, α = 0.05
Sample standard deviation, s = $21
First, we design the null and the alternate hypothesis
We use one-tailed t test to perform this hypothesis.
Formula:

Putting all the values, we have

Now,
Since,
The calculated test statistic is greater than the critical value, we fail to accept the null hypothesis and reject it. We accept the alternate hypothesis.
Conclusion:
There is enough not evident to support dealer's claim that his sales is no more than $80.