Answer:
Shirley's water bottle holds 4 quarts when it is half full.
Step-by-Step-Explanation
If the second bottle is one-third full and holds 2 quarts, then it means that it can hold a total of 2 / (1/3) = 6 quarts when completely full.
Let's call the amount of water in Shirley's bottle "x" quarts. We know that when she pours it into the second bottle, it becomes completely full with 6 quarts. So we can set up the equation:
x + 2 = 6
Solving for x, we get:
x = 6 - 2
x = 4