If the ratio is 4:1 , pencils being the 4 and erasers being the 1, there are 4 pencils for every 1 eraser. Now, substitute the 4 pencils for 20 pencils. How do you get to 20 from 4? multiply by 5. You multiply the 4 pencils by 5 to get 20, and u multiply the 1 eraser by 5 to get 5, so there are 20 pencils: 5 erasers