The mean grade in Mathematics of Grade 11 students from school A is 83 with standard deviation of 4, while the mean grade in Mathematics of Grade 11 students from school B is 87 with standard deviation of 5. Using 49 samples from school A and 64 from school B, what is the probability that the mean grade of students from school B exceeds the mean grade of students from school A by at most 6?

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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.