From the condition, the cold water facet is 4 times as productive as the hot water facet.


So, when both facets are turned on, it is equivalent as if 5 hot water facet are turned on (from the inflow rate point of view).


Thus, 5 hot water facets will fill the bathtub in 3 minutes - hence, one hot water facet will fill it in 15 minutes.


It implies, that cold water facet will fill the bathtub in 4 times faster, i.e. in   minutes =  minutes = 3 minutes and 45 seconds.