Do a ratio dude:
12.2 19.5
------- = ------
200 x
Solve, and you get that x is approx equal to 320 mg.
Decomposers are necessary to the food chain because they break down the results of the food chain into the basic elements that producers can then use to begin the process over again. Decomposers include groups of bacteria that help to speed up the decaying process of dead plants and animals.
Hope that helped! =)
