Cathy saved $320 last month. This month, she saved $377.60. What is the percent increase for the amount of money Cathy saved?

At the first you should calculate the difference amount between two month:

$ 377.60-$320=$57.60

And then you divide $57.60 by $ 320

like $57.60 / $320 = 0.18=18%

So 18% is the answer