The unit rate of the relation between water pumped and time will be the amout of water pumped in one minute so:
[tex]\frac{28}{2}=14[/tex]this means that each minute is pumped 14 gallons of water
now we can calculate the water pumped in 3/2 minutes so
[tex]14\cdot\frac{3}{2}=21[/tex]So in total the addition of the water pumped in 12 minutes plus 3/2 minutes will be the water pumped in 15/2 minutes
[tex]14\cdot\frac{15}{2}=105[/tex]