Answer:
a) 0.023
b) 0.286
c) 10 students will be unable to complete the exam inthe allotted time.
Step-by-step explanation:
We are given the following information in the question:
Mean, μ = 80 minutes
Standard Deviation, σ = 10 minutes
We are given that the distribution of time to complete an exam is a bell shaped distribution that is a normal distribution.
Formula:

a) P(completing the exam in one hour or less)
P(x < 60)

Calculation the value from standard normal z table, we have,

b) P(complete the exam in more than 60 minutes but less than 75 minutes)

c) P(completing the exam in more than 90 minutes)
P(x > 90)


Calculation the value from standard normal z table, we have,

15.87% of children of class will require more than 90 minutes to complete the test.
Number of children =

Approximately, 10 students of class will require more than 90 minutes to complete the test.