Ten big water bottles can hold 20 gallons of water.
When writing unit rates, y goes over x. However, in the problem above, x represents the water bottles and y represents the gallons of water.
4/8 (x, y) -> 8/4 (y, x)
8/4 \ 4/4 = 2/1
Now, you know the constant rate of change. There are 2 gallons of water for every 1 big water bottle. Simply multiply 2 by 10.
Since you are looking for how many gallons of water in ten water bottles, multiply 2 gallons of water (I assume you now know that there are 2 gallons of water in every 1 big bottle) by the number of water bottles. In this case, 10.
