Question

The ratio of pencils to erasers is 4:1 if there are 20 pencils how many erasers are there

Answer 1

Well, think of it like a proportion. Let's say pencils is P and erasers is E. This means, P/E = 4/1. But, you need to find the number of erasers when you have 20 pencils. So, then we put 4/1 = 20/E because without the numbers this means P/E = P/E. From here you can think of this in 2 ways.
1st way:
You can think about what happens to 4 to make it 20. (Or divide 20 by 4.) You should get that 4x5 = 20. So, since it's a proportion, what happens to the top must happen to the bottom. That means you must multiply the bottom by 5 as well to get 5 erasers when there are 20 pencils.
2nd way:
You can cross multiply the proportions. This means, multiply 4xE and 20x1 and set them equal to each other. So, 4E=20. solve for E (or divide 20 by 4) and you get 5. That's the answer because E in this equation equals the new number of erasers when you have 20 pencils.
The answer is 5 erasers.

Answer 2

The answer to your question is 5 erasers