I don't get this math question (6th grade math, common core) *Ratio Table*

This is for my son, I don't get nor does he, here's the question.

"Before Leaving to visit Mexico, Levant traded 270 American dollars and received 3,000 Mexican pesos. When he returned from Mexico, he had 100 pesos left, How much will he receive when he exchanges these pesos for dollars?"

There's also a chart that reads

American Dollars: 270 (blank) (blank)
Mexican Pesos: 3,000 (blank) 100

The blanks are what he need to put there, also, can someone also explain to my son and I what ratio tables are? They are working on this for 2 weeks. Please help, thank you!

it would be 270 over 3000 is equal to ? over 100. So you would cross multiply 100 times 270 and get 27000. Then you would divide 27000 by 3000 to get 9. So he would have $9 American Dollars back and that is equivalent to 100 pesos. Now for the middle you would have to have at least one number. So there you would possibly just pick any number. Ratios are easy you take the numbers just like the chart and turn them into fractions. Then you would just cross multiply and divide.