Step-by-step explanation:
7 people = 280 liters
1 p = 40 liters
50 p = 40 x 50
50 p = 2000 liters
hope it helps.
Also, I think that Brainly is an awesome app, but there's an app which is doing great work for me in maths, named Gauthmath. I will suggest it. Video concepts and answers from real tutors.