If it takes the cold water faucet x minutes to fill 1 tub, then its flow rate is (1 tub)/(x min).
The hot water faucet takes 4 times as long, so its flow rate is (1 tub)/(4x min).
Together, the total flow rate is (1 tub)/(3 min), so we have
[tex]\dfrac1x+\dfrac1{4x}=\dfrac13[/tex]
[tex]\implies\dfrac5{4x}=\dfrac13[/tex]
[tex]\implies15=4x[/tex]
[tex]\implies x=\dfrac{15}4\,\text{min}=3\,\text{min}\,45\,\text{sec}[/tex]
