The value of a is between 0 and 1.
Answer:


Step-by-step explanation:
For this case we have the following data given:
2.3 3.1 2.8
1.7 0.9 4.0
2.1 1.2 3.6
0.2 2.4 3.2
Since the data are assumedn normally distributed we can find the standard deviation with the following formula:

And we need to find the mean first with the following formula:

And replacing we got:

And then we can calculate the deviation and we got:

Answer: 1 hour 20 minutes
Step-by-step explanation:
Given : The old machine could shred a truckload of paper in 4 hours.
Rate of work done by old machine = 
The new machine could shred the same truckload in 2 hours.
Rate of work done by new machine = 
Let 't' be the time taken by both of them working together , then we have the following equation:-

Since 1 hour = 60 minutes
Then, 
Hence, it will take 1 hour 20 minutes to shred the same truckload of paper if Ron runs both shredders at the same time.