Since you need to travel 850 miles, and you can travel 20 miles per gallon, you should divide 850 by 20 to get the number of gas cans needed. In doing so, you get 42.5 gas cans, but since it may be asking for whole cans rather than part of a gas can, you might want to write 43 instead. If you were to round down to 42, you would not have enough gas to finish the trip, so you should round up to 43. So, it is up to you if you want to write 42.5 or 43.
