I need help I need help

What is the best way to answer questions with a x expression in the question. for example 3x + 10x=?

Which expression gives the exact value of x?

At a pizza shop 70% of the customers order a pizza 25% of the customers order a salad and 15% of the customers order both a pizza and a salad if s customer is chosen at random what is the probability that he or she orders either a pizza or a salad