$3.20$ dollars = $320$ cents  $(320-40\times6)\div2=40$ cents 
Six pencils and two erasers cost $$3.20$$ dollars. One pencil costs $$40$$ cents. How much does one eraser cost? (Adapted from 2000 Math Kangaroo Problem, Level 3-4, Question \#5) 

  - $$20$$ cents 
  - $$30$$ cents 
  - $$40$$ cents 
  - $$50$$ cents 
  - $$60$$ cents 