Let X be a random sample representing the score of a student in the American Chemical Society Examination.
P(450 < x < 500) = P((450 - 500)/(90/sqrt(25)) < z < (500 - 500)/(90/sqrt(25))) = P(-2.778 < z < 0) = P(z < 0) - P(z < -2.778) = P(z < 0) - [1 - P(z < 2.778)] = P(z < 0) + P(z < 2.778) - 1 = 0.5 + 0.99727 - 1 = 0.49727 = 49.7%
First we have to find 25% of 900 pounds (i hope that's the right currency). so 900 x .25 = 225
so we subtract 225 from 900 and get 675 pounds.
to find sales tax we have to find 6% of 675, so 675 x .06 = 40.50
675 + 40.50 = 715.5 pounds
