normal distribution

sir this is a question from the acca site the percentage probability that a student will score more than 90%in an accounting exam is 2.28%. the marks scored from the accounting exam follow a standard normal distribution with a mean mark of 70%. what is the standard deviation for the distribution ? so the answer is the probability that a students mark is more than 90% is 2.28% using the standard normal distribution table , work backwards to find the z score . Z = 0.50 - 0.0228 = 0.4772 so from the table = 2 z score = x - the mean / standard deviation 2 = 90 - 70 / std dev so the answer is 10 which is the standard deviation . dear sir according to my knowledge and as per what i learnt It is simply a measure of the average spread of the distribution. The lower the standard deviation then the less the spread is (if it is 0 then there is no spread at all). The higher the standard deviation then the more the spread. For example, suppose a data set has just two observations: 10 and 30. The mean here is 20 and the standard deviation will be 10 as both observations are 10 units away from the mean. so sir the problem i have is in the above question the standard deviation was 10 but i cannot really understand what is the use of knowing this figure 10 .. because the mean is 70 and then standard deviation is 10 is it something to do with knowing the below "10 lower than 70 is 60 and 10 higher than 70 is 80" sir i did watch the lectures and i do calculations very good but the theoratical part is lacking in me