Salt Water of Concentration 0 $f ( \mathrm { t } )$

Salt water of concentration 0.5 pound of salt per gallon flows into a large tank that initially contains 220 gallons of pure water. If the flow rate of salt water into the tank is 4 gal/min, find a formula for the salt concentration $f ( \mathrm { t } )$ (in lb/gal) after t minutes.

A) $f ( t ) = \frac { t } { 220 t + 55 }$
B) $f ( t ) = \frac { 4 t } { 2 t + 110 }$
C) $f ( t ) = \frac { t } { 2 t + 110 }$
D) $f ( t ) = \frac { 4 t } { 220 t + 55 }$
E) $f ( t ) = \frac { 4 t } { 2 t - 110 }$