Assuming that the number of marks scored by a candidate is normally distributed, find the mean and the standard deviation, if the number of first class students(60% or more marks) is 25, the number of failed students(less than 30%marks) is 90 and the total number of candidates appearing for the examination is 450.
Help me solve . Thanks

You have two unknowns (mean and stddev) and two equations. Just have to transform the numbers to proportions, thats where you need the total number of participants.