In 4 month, 104 gallons were used(determine gallons per month)

All you have to do is divide 104 by 4! The awnser is 26

5x=number of red tents
3x=number of blue tents
x=number of white tens

we can suggest this equation:

5x+3x+x=1350
9x=1350
x=1350/9=150

number of red tents=5x=5(150)=750
number of blue tents=3x=3(150)=450
number of white tents=x=150

C.  750 red, 450 blue, 150 white.