Knowing how to find the gas mileage for a vehicle is a valuable skill. You simply divide the number of miles driven since the last fill up (likely on the car's trip odometer) by the number of gallons of gas used (on the gas pump and on your receipt). However, it may be much easier to round off one or both of the numbers in order to do the problem with ease.
If you drive a car 282 miles while it used 6.9 gallons of gas you can easily convert the problem to 280miles divided by 7gallons, which is 40 miles per gallon.
If is a car is driven 623 miles and used 24.2 gallons of gas, you may want to convert the problem to 625 divided by 25, which is 25. It is helpful to know the squares of numbers. 11² = 121, 12² = 144, 13² = 169, 14² = 196, 15² = 225, 16² = 256, 17² = 289, 18² = 324, 19² = 361, 20² = 200
Strive to be able to do these problems as 'mental math' problems and then strive to increase your speed. Good luck and enjoy the challenge!