Step-by-step explanation:
x = number of bottles Paula sold
y = number of bottles Pedro sold
x + y = 72
x = 72 - y
x/y = 3/5
(72 - y)/y = 3/5
5(72 - y) = 3y
360 - 5y = 3y
360 = 8y
y = 45
x + y = 72
x + 45 = 72
x = 27
so,
Paula sold 27 water bottles, and Pedro sold 45.