Answer:
a) 
b) 
And replacing we got:

And the deviation would be:

c) 
And we can use the z score formula given by:


And replacing we got:

And using the complement rule we got:

Step-by-step explanation:
For this case we have the following info:
Freshmem

Sophomores

We select a sample of 40 for both groups 
Part a
The expected value for the difference is given by:

Part b
We know that the sample mean follows this distribution:

And we want the distribution for 
And we need to find the variance with the following formula:

And replacing we got:

And the deviation would be:

Part c
For this case we want this probability:

And we can use the z score formula given by:


And replacing we got:

And using the complement rule we got:
