The additional amount of gallons of petrol that would be needed to complete the journey is; 4 gallons
A) 1 gallon is required for 30 km drive. Thus;
420 km drive will require; 420/30 = 14 gallons
Since he had 10 gallons already, then additional gallons required = 4 gallons
B) We are told that  a gallon of petrol cost Ghc 5.50​. Thus;
14 gallons would cost = 420 * 14 = Ghc 5880
Read more about proportion at; https://brainly.com/question/1781657
#SPJ1