A professor at a local university noted that the grades of her students were normally distributed with a mean of 78 and a standard deviation of 10.

The professor has informed us that 23.89 percent of her students received grades of A. What is the minimum score needed to receive a grade of A?

(Please, use only one decimal Place)

Graph a should be your answer

So the first thing we do is to multiply the top equation by 3. the reason i want to do this is i notice there will be a 6y on both equations and can possibly add/subtract them. so we get 6y = -3x + 27. adding that and the second equation we get 6x = 27-15 which is 12. divide by 6 and we get 2. ∴ x = 2. substituting 2 into the first equation we get 2y = -2 + 9 which is 7. divide by 2 and we get 7/2. ∴ y = 7/2, x = 2.