Answer: 32 ounces.
Explanation:
We can think of this as a linear equation. 
y = a*x + b 
where y is the amount of water in the bottle, a is the rate at which loses water, x is the number of seconds and b is the initial amount of water. 
We have two pairs: 
2 seconds - 28 ounces left or (2, 28) 
6 seconds - 20 ounces left or (6, 20) 
Now we can write the value of a as: 
a = (20 - 28)/( 6 - 2) = -8/4 = -2 ounces per second. 
Then our function is: 
y = -2x + b 
and we know that when x = 2, y = 28, so we have: 
28 = -2*2 + b = -4 + b 
b = 28 + 4 = 32 
the initial amount of water in the bottle is 32 ounces.