Answer=875miles    17.5miles per gallon
 350miles    x miles
________=_________
20gallons   50gallons
cross multiply
20x=17500
divide both sides by 20
x=875
875 miles can be traveled with 50 gallons of gas.
To find gas milage, which is essentially miles per gallon, simply divide the number of miles by the number of gallons
350miles  20gallons
350/20=17.5 miles per gallon