The given information tells us that for every one flight Muddville has, Fly Low airlines will have 3. This means they have a 3:1 ratio
X = fly low airlines flight, Y = muddville airlines flights
3X + Y = 16
3 + 1 = 4
6 + 2 = 8
9 + 3 = 12
12 + 4 = 16
This means in one day, Fly low airport will have 12 flights while Muddville airport will have 4