Answer:
189
Step-by-step explanation:
If mark uses 19 gallons to travel 399 miles, that means he was able to travel 21 miles per gallon.
We get this by taking 399 divided by 19.
So if mark travels on 9 gallons of gas and goes 21 miles per gallon, he will have traveled 189 miles.
We get this by taking 21 times 9