ANSWER and EXPLANATION
We want to identify how the amount of aluminum foil used in a solar oven affects the speed at which the food cooks.
A solar oven works by using the aluminum/tin foil in the cooker to reflect the sun's rays and heat into the box. In this process, the aluminum foil helps convert some of the light energy into heat energy.
Since the amount of aluminum foil used directly determines how much of the sun's energy is reflected, it implies that the use of more aluminum foil will result in more energy being reflected and concentrated on the food, hence, cooking the food faster.
Conversely, less aluminum foil would imply less energy reflection and concentration, hence, slowing down the rate at which the food is cooked.