Let x be a random variable representing the number of skateboards produced
a.) P(x ≤ 20,555) = P(z ≤ (20,555 - 20,500)/55) = P(z ≤ 1) = 0.84134 = 84.1%
b.) P(x ≥ 20,610) = P(z ≥ (20,610 - 20,500)/55) = P(z ≥ 2) = 1 - P(z < 2) = 1 - 0.97725 = 0.02275 = 2.3%
c.) P(x ≤ 20,445) = P(z ≤ (20,445 - 20,500)/55) = P(z ≤ -1) = 1 - P(z ≤ 1) = 1 - 0.84134 = 0.15866 = 15.9%
Converting a decimal to a mixed number is very simple when you get the hang of it. Since 8 is before the decimal, that is your number in the mixed number. For the fraction part, it's 77/100. The denominator is 100 because when you count the decimal places, it goes to hundredths. 77/100 is already in simplest form, so your final answer is 8 77/100.
So 75 points is your 100%. You can do 60 over 75 is equal to X over 100. You divide your numerator(60) by your denominator(75) to get 85.7142857%, or to round to your nearest percent, your answer would be roughly 86%.
The double angle identities are
Then
The second identity together with the Pythagorean identity, 
, gives us another equivalent expression:
so
