A Professor has 50 students in his class and grades all of them on their English Literature coursework. The mean score is 60 out of 100 and the standard deviation (in other words, the variation in the scores) is 15.

Having looked at the performance of the Professor's class, one student, Sarah, has asked the Professor if, by scoring 70 out of 100, she has done well. Bearing in mind that the mean score was 60 out of 100 and that Sarah scored 70, then at first sight it may appear that since Sarah has scored 10 marks above the 'average' mark, she has achieved one of the best marks. However, this does not take into consideration the variation in scores amongst the 50 students (in other words, the standard deviation). After all, if the standard deviation is 15, then there is a reasonable amount of variation amongst the scores when compared with the mean.

Whilst Sarah has still scored much higher than the mean score, she has not necessarily achieved one of the best marks in her class.

Using a standard normal distribution (and therefore calculating her z-score) to get your answer, please indicate how many standard deviations above the mean Sarah's score was on the English Literature coursework.