To find the mean, add the 5 daily values and divide by 5. The sum is $8+10+7+9+11=45$. Then compute $\frac{45}{5}=9$. So the mean number of students per day is $9$.

The mean of 5 values is 20, so the total of the 5 temperatures must be $5\cdot 20=100$. The four known temperatures add to $18+21+19+22=80$. Therefore, the missing temperature is $100-80=20$. So, $x=20$.

The mean of a data set is found by dividing the total by the number of values:

Here, the mean is $12$ and there are $5$ values, so

Multiply both sides by $5$:

So the expression that represents the total is $12\cdot 5$.

The range of a data set is the greatest value minus the least value. In this data set, the least value is $8$ and the greatest value is $12$. So the range is $12-8=4$. Therefore, the correct answer is $4$.

Since the mean of 5 numbers is 42, their total must be $5\cdot 42=210$. The four known numbers add to $35+40+47+50=172$, so the remaining number is $210-172=38$. Check that the median is still 40: the five numbers are $35,38,40,47,50$, so the middle value is 40. Also, all 5 numbers are different integers. Therefore, the remaining number of visitors was $38$.