The answer to this is 400 pounds. Computing for this is going to be easy. You simply need to take a look at the terms that are used. The keyword here is total which means that you need to know all of the combined weight of the kegs of nails.
The best process to use for this is multiplication which means that your equation will be 4x100 = 400. It can be very useful to become familiar with the multiplication table especially if you would be required to multiply other numbers in the future.
You may also use addition for this if in case you want to. The equation for this will become 100+100+100+100 = 400.