On the nav. computer put 22 on the inner main scale next to the U.S.gal. index, go to the imp.gal. index and note fuel consumption is 18.2 IG per hour.  

Put the 60 (minutes) on the inner main scale next to 18.2 (IG/hr) on the outer main scale.  

Go to 83 (IG – total fuel available) on the outer main scale and read next to it the fuel endurance: 272 minutes = 4 hours 32 minutes.