Sarah's english test had 90 questions and she got 18 questions wrong. what percent of the questions did she get correct?

So,

Sarah got 72/90 questions correct.

Simplify 72/90
72 = 2 * 2 * 2 * 3 * 3
90 = 2 * 3 * 3 * 5

Cross out "ones"
2 * 2 over
5
4/5 = 80%

Sarah got 80% of the questions correct

90-18=72 questions (answered correct)
72/90=0.8
To get the percent, you have to move the decimal point over two places, so 80%

If you need an equation, it would be y=2x+8, y would be how long it grew, x being weeks.if you need how long he was asleep, it would be 4 weeks if he started with no beard, the equation would be x=8÷2if he did have a beard before hand, the equation would be 2w+x=8