Are you guys bored? For starters you won't multiply it by 18. If you do that then you are saying you just have 18 cars.
Take a cube for example. The cube is 1 pound and is 1 foot square. If you double it to 2 feet square how much does it weight.
8 pounds not 2. Why? To double the mass it will take 8 cubs length, width, and height.
Second 5 pounds sounds realy heavy for that model. The specific gravity of plastic is about 1, the specific gravity of iron is about7.9. Which means if you have 1 pound of plastic the exact same thing in iron would be 7.9 pounds.
and so forth
you need to find out what parts are what like glass, rubber, steel and cloth.
each of the plastic pieces will have different weights.
so just guessing on a calculator won't get you there.