Answer:
a) 
b) 
c) 
Step-by-step explanation:
For this case we know that the mean for the random variable of interest is  and the variance
 and the variance  so then the deviation would be
 so then the deviation would be 
The z score is given by thsi formula:

Part a
We want this probability:

And if we find the z score we got:

And we can find this probability: 
Part b
We want this probability:

And if we find the z score we got:

And we can find this probability: 
Part c
We want this probability:

And if we find the z score we got:

And we can find this probability: 