Underdogg

Hey everyone, It seems to be a recurring problem for me, but whenever I'm presented with a question that involves "doubling the value or halving the value" of a variable in an equation, I can never get it right. In case I haven't made it clear enough, say you had the formula for Newton's Law of Gravitation: F = -GMm/r2 A typical question would be: What would be the new value of the result (from a previous question), if the value of "r" was quartered? Any help is greatly appreciated. Thank you