please help me to solve these questions correctly and asap. and please show calculation steps.

Question 3 The marks obtained by students taking the Probability & Statistics module is normally distributed with a mean of 80% and a variance of 225. If 25 students are chosen, calculate the: a) probability that the students obtain percentage score between 75 and 90. [3 marks] b) percentage of students obtained at least than 76%. [3 marks] c) Estimate the number of students who will obtain a perfect score. [4 marks]