In a random sample of 80 students in first grade the main hours of sleep per night was 9.4 with a standard deviation of 0.41 in a random sample of 50 students in 12th grade the main hour of sleep per night was 7.5 with a standard deviation of 0.52. which interval is a 99% code finance interval for the difference between the main hours of sleep.
(1.75 h, 2.05 h)
(1.89 h, 1.91h)
(1.43 h, 2.37h)
(1.68 h, 2.12 h)

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.