Question

How do you solve 0.004 dm + 0.12508 dm?

Answer 1

0.004 of something added to 0.12508 of the same thing
adds up to 0.12908 of it.

The thing could be a glass of water, a sheet of paper,
a pound of ground beef, a gallon of gas, or a snowball.
In this problem, it just happens to be a dm.