Yes, they are. They cannot happen both. Every time that you roll a cube you can get only one number, then if you roll a 4 you cannot roll a 3 at the same tiem, and if you roll a 3 you cannot roll a 4 at the same time.
Mutually exclusive events cannot happen at the same time.
So what you do is find 25% of 80% of 100%. So, 8 out of 10 kids have a fever, and 2 out of 8 kids have a sore throat. So you would have 5 out of 10 kids have a sore throat, and a fever