Answer: D) 125%
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In 1985, there were 20 students
In 2005, there were 45 students
This is a change of 45-20 = 25 students.
This amount of change is divided by the original amount 20 to get 25/20 = 1.25 and converts to 125% when you move the decimal point 2 spots to the right.
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If you want, you can use the formula below
x = old value = 20
y = new value = 45
z = percent change
z = [ (y-x)/x ] * 100%
z = [ (45-20)/20 ] * 100%
z = (25/20) * 100%
z = 1.25*100%
z = 125%
The z value being positive means we have a percent increase.
It's possible to have over 100% increase since 100% of 20 is 20. Having a 100% increase means we increased by 20. Since we increased by more than 20, this means the increase must be over 100%.