You have eaten 5.85 oz of the cereal.
Step-by-step explanation
The weight of the box of chocolate cereal at the time of buying was 13 oz.
Now, you've eaten 45% of a box.
So for finding the amount of the cereal that you have eaten, you need to find 45% of the 13 oz.
So, 45% of 13 oz

Thus, you have eaten 5.85 oz of the cereal.