Pls answer and explain

Let X be a random variable representing the mean gray scale of each pixel.
P(110 ≤ X ≤ 140) = P(X < 140) - P(X < 110) = P(z < (140 - 125)/15) - P(z < (110 - 125)/15) = P(z < 1) - P(z < -1) = P(z < 1) - [1 - P(z < 1)] = 2P(z < 1) - 1 = 2(0.84134) - 1 = 1.68268 - 1 = 0.68268

Number of pixels that have gray scale of between 110 and 140 = 0.68268 x 1000000 = 682680 = approximately 680,000