270 american dollar and recevied 3000 pesos when he returned from mexico he had 100 pesos left. how many will he recevie when he exchanges these pesos for dollars

To solve this problem we can use simple proportion
If 
270$ 3000 pesos
x $  100 pesos               (x$ means that we dont know how much)
Now we crossmultiplying to get proportion
x*3000=270*100
Now we just to solve eq
3000x=27000  /:3000
x=27000:3000
x=9$ - its the result

The variables should add up to equal 1