When both facets are turned on, they fill 1%2F4 of the bathtub volume per minute.
When only cold facet is turned on, it fills 1%2F12 of the bathtub volume per minute.
It means that the contribution of the hot facet is the difference 1%2F4 - 1%2F12 = 3%2F12 - 1%2F12 = 2%2F12 = 1%2F6 of the bathtub volume per minute.
Therefore, it will take 6 minutes for the hot facet to fill the bathtub working alone.
