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
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: 