If you want to solve this, you simply need to divide both sides by 40. You do this so that there will be one dollar (40/40 = 1) and you can then determine the number of corresponding minutes.
Since you know 40 divided by 40 is going to be one (the dollar you're looking for!), you can simply divide 160 by 40 to get four.
Therefore the answer is four minutes per dollar!
