public class Bottle
{
    private final double capacity;
    private double amountRemaining;

    public Bottle(double capacity)
    {
        this.capacity = capacity;
        amountRemaining = capacity;
    }

    public double updateAmount(double amountUsed)
    {
        final double MIN_PERCENT_LEFT = 0.25;

        amountRemaining -= amountUsed;

        if(amountRemaining < capacity * MIN_PERCENT_LEFT)
            amountRemaining = capacity;

        return amountRemaining;
    }
}