Question 413154
Here's the formula that I like to use to solve problems like these that my math teacher showed me:
%        part
-    =   ----
100      whole

So, for this question, it would be:

x        28
-    =   --
100      35

Then, you would use cross products, and you would create an equation like so:

35x=100*28
And then solve!
35x=2800
x=2800/35
x=80%