You can find many of your answers by visiting
http://www.convert-me.com/en/ But I will help as much as I can.
1) Converting miles into kilometers:
1 mile equals 1.60934400579467 kilometers so multiply the number of miles you are trying to convert by 1.60934400579467 and you have the equivalent in kilometers.
2a) Converting pounds in grams:
There are 453.592374495299 grams in 1 pound so multiply the number of pounds you are trying to convert by 453.592374495299 and you have the equivalent in grams.
2b) Converting pounds to kilograms:
There are 0.453592374495299 kilograms in 1 pound so multiply the number of pounds you are trying to convert by 0.453592374495299 and you have the equivalent in kilograms.
3) Converting gallons to liters:
There are 3.78541181779072 liters in 1 gallon so multiply the number of gallons you are trying to convert by 3.78541181779072 and you will have the equivalent in liters.
4) 20 miles / gallon = __ kilometers / liter
Our constants are: 1 mile = 1.60934400579467 kilometers and 1 gallon = 3.78541181779072 liters. Plugging those into our desired input gives us:
20 miles / gallon multiplied by 1.60934400579467 kilometers / mile multipled by 1 gallon / 3.78541181779072 = 8.502874 kilometers / liter