You can answer any percent question using the "is over of" method. This is a proportion - an equation where one ratio equals a second ratio. One of the proportions will always be the percent over 100. In this case that is 25/100.
The second proportion will be the number associated with "is" in the question (in this case, 70) over the number that follows the word "of" in the question (in this case, an unknown so we represent it with a variable); 70/x
So the proportion is 25/100 = 70/x
To solve for the unknown (in whatever position it happend to be in for a specific problem) you cross multiply, to obtain: 25x = 100*70.
Solving for x gives x = 7000/25 = 280. So 70 is 25 percent of 280.
This is a perfect example of what I was discussing in my answer to the question "Applications of algebra in real life" http://hubpages.com/question/35620/applications-of...
You may know how to answer the question "What is 25% of 100?" without using algebra; or maybe even "What is 25% of 280?" But when the question is turned around as this one was, you're at a loss as to how to find the answer. Well, as I always say (not really, but it sounds good ), Algebra to the rescue!