If the gas tank of Dave's car has a capacity of 12 gallons and was 3/8 full, how much did he spend to fill the tank to capacity at $2.50 per gallon?

To determine how much Dave spent to fill his gas tank to capacity, we first need to figure out how many gallons he needs to add to the tank. Since the tank has a total capacity of 12 gallons and is currently 3/8 full, we calculate the current amount of gas in the tank.

First, we find out how many gallons are in the tank when it's 3/8 full:

[

\text{Current gallons} = \frac{3}{8} \times 12 = 4.5 \text{ gallons}

]

Next, we determine how much more gas Dave needs to add to fill the tank to its full capacity:

[

\text{Gallons needed} = \text{Total capacity} - \text{Current gallons} = 12 - 4.5 = 7.5 \text{ gallons}

]

Now that we know he needs to add 7.5 gallons, we calculate the cost to fill the tank at the price of $2.50 per gallon:

[

\text{Total cost} = \text{Gallons needed} \times \text{Price per gallon} = 7.5 \times 2.50 = 18