Since each keg weighs 100 pounds, it will be safe to assume that four kegs will be equal to 400 pounds. You can choose to multiply or to add. No matter what method you will decide on using, you will still reach the same results. For example, if you want to multiply, you can just do 4x100.
The answer to this will be 400. If you want to add, just do this equation: 100+100+100+100 = 400. Once you already have the answer, do not forget to place the unit as this can sometimes be enough to mark your answer wrong especially if you have forgotten to place the proper unit.