Could someone explain the steps to me? i dont get it

Stefania pours 2 liters of orange juice and 1.5 liters of pineapple juice into a punch bowl. how many liters are in the punch bowl?

Agatorade cooler has 36 cups of water. if a serving is 3/4 of a cup how many servings are in the cooler?

While on a train trip, marsha noticed that there were 35 people in her train car. after the next stop, there were only 21 people in her car. if p represents the percent decrease in the number of people in marsha's train car, what is p?