I'm practicing for SAT, and encountered this question:
The graph below shows the percentage of American households that owned landline telephones from 2000 to 2006.
According to the data above, the percentage of American households with landline telephones in 2001 was what percent greater than the percentage of American households with landline telephones in 2005?
I know that such a question should be solved like this: Difference / Original.
So I solved it like this: (Percentage in 2001 - Percentage in 2005) / Percentage in 2005.
= (80 - 60) / 60 = 20 / 60 = 0.33 = 33%.
But then when I reviewed the answers the right answer was: 25%. It said that 80 is the original number and not 60 so it becomes: 20 / 80 = 0.25 = 25%.
My question is: How to know which one is the original number? (it's obvious in some questions but here it's so confusing).
The answer you calculated is correct. In context, your answer allows us to answer the question as: "According to the data above, the percentage of American households with landline telephones in 2001 was $33.3$ percent greater than the percentage of American households with landline telephones in 2005". Which makes sense as $80=60\times\left(1+\frac{33.3}{100}\right)$. But with the given answer we instead have $60=80\times\left(1-\frac{25}{100}\right)$ which does not answer the given question.