There are two main ways to solve this. Many students who aren't strong in percents prefer the ratio method.
64.2/100 = ?/84
You can then cross multiply to get 64.2*84 = 100*?. Doing the math (64.2*84, then divide by 100) you get 53.928.
The more sophisticated method would set this up as an equation. Remember, percents are parts out of 100, so 64.2% is really .642
(.642)*84 = (do the math) = 53.928
If you can understand the second method, it's much more "calculator friendly" in the long run, and will be more useful.
