To find out how much each pencil cost, you can set up an equation.

1. Start with the total amount you had: $24.
2. Subtract the amount you had left after buying the pencils: $24 - $20 = $4.

So, the total cost for the two pencils is $4.

3. Divide by the number of pencils:

$\text{Cost per pencil} = \frac{4}{2} = 2$

Each pencil cost $2.

Would you like more details, or do you have any questions?

Here are some related questions you might find interesting:

1. If you had $30 instead of $24, how much would you have left after buying two pencils costing $2 each?
2. If each pencil cost $3, how much would you have left after buying them with $24?
3. How much would you have to start with if each pencil cost $5 and you wanted $20 left after buying them?
4. If you had $24 and bought four pencils for $2 each, how much would you have left?
5. What would happen to the total cost if the price of each pencil increased by $1?

Tip: When solving word problems, break down the information step by step to form equations that you can solve easily.