Final answer:
The oven needs to be on for 39 minutes in total.
Step-by-step explanation:
To find out how long the oven needs to be on, we need to add up the time it takes to heat the oven and the time it takes to bake the batches of cookies.
First, it takes 15 minutes to heat the oven.
Next, it takes 8 minutes to bake each batch of cookies, and since Ethan is baking 3 batches, it will take a total of 8 x 3 = 24 minutes to bake all the batches.
Therefore, the oven needs to be on for 15 minutes (heating time) + 24 minutes (baking time) = 39 minutes in total.