13. Marbles come in two sizes: 2-inch diameter and 3-inch diameter. The 3-inch diameter marbles cost 50 cents more than the 2-inch marbles. Jorge buys 20 3-inch marbles for a science project and spends exactly $16. Find the cost of a 2-inch marble.
Let's say the cost of a 2-inch marble is x cents.
The cost of a 3-inch marble is then x + 50 cents.
Jorge buys 20 3-inch marbles, so he spends 20 * (x + 50) cents on the 3-inch marbles.
He spends exactly $16, which is equal to 1600 cents.
So, we have the equation: 20 * (x + 50) = 1600.
Dividing both sides of the equation by 20, we get: x + 50 = 80.
Subtracting 50 from both sides of the equation, we get: x = 30.
Therefore, the cost of a 2-inch marble is 30 cents. Answer: \boxed{30}.