If a killer whale is feeding at the fourth trophic level, how much phytoplankton mass is required to add each gram of new mass to the whale? assume that 10% of the energy at one trophic level is transferred to the next level.

In order to add one gram of mass, the whale must consume a sufficient amount of energy. The energy transferred to the fourth trophic level is 0.1⁴ = 0.0001
We divide the mass to be gained by the energy obtained per unit mass, to find the total mass to be eaten:
1 / 0.0001 = 10,000 grams of phytoplankton must be consumed to add one gram of mass.