Assimilation efficiency quantifies the ratio between the organism's used energy for growth and production of new cells and tissues, and others.
We let x be the amount of substance assimilated.
AE = x / 10,000
From the given, AE is equal to 0.5. Substituting,
0.5 = x/10,000
x = 5,000
Production efficiency on the other hand is the ratio of the amount of substance used for production compared to the assimilation. Mathematically,
PE = 2,000 / x = 2,000/5,000
PE = 0.4
This is equivalent to a percentage of 40%.