When can you and when can you not combine partial derivatives

The system of equations graphed below has how many solutions?

For one history test, keith had to answer 40 questions. of these 40 questions, keith answered 28 of them correctly. what percent did keith get on his history test? round your answer to the nearest tenth if necessary

Arestaurant needs to buy thirty-seven new plates. if each box has five plates in it, how many boxes will they need to buy?