HELP: Normal distribution probability problem

I have the following problem:

*A certain small freight elevator has a max. capacity C, which is Normally distributed, with mean 400 lbs., and standard deviation 4 lbs.*

The weight of the boxes being loaded into the elevator is a R.V., with mean 30 lbs., and standard deviation 0.3 lbs.

How many boxes may be loaded into this elevator before the probability of disaster exceeds 0.2?

I have some idea of how to get started, but I can't seem to find the right formula. Can any one help? Thanks in advance, any help is greatly appreciated.

Mikhail