Ratio of gas to oil = 50 : 1
Quantity of oil = x gallons
Quantity of gas = 20 gallons
Then using the given ratio,
50 : 1 = x : 20
By cross mutliplying we get
x=1000 gallons.
Therefore you need 1000 gallons of oil for 20 gallons of gas.
Hope this helped,
Best of luck.