1. Yep, that's right
If you check the discriminant of the expression you get , so there are no real solutions, and hence V never reaches 0. The tank never empties completely.
3. You want to find the lowest value of V. Therefore, you need to find the turning point of the graph .
Just complete the square here:
Hence, the turning point is located at (8, 36). So 64 litres flow out of the tank and 36 litres remains.
4. Simply solve for t. Fortunately having it in 'complete the square' form makes it a lot easier to solve.
Notice since the parabola curves upwards again you will get two values of t for the one value of V.