Answer
Net secondary production is 20J.
Production efficiency is 40%.
Work Step by Step
The net secondary production is what's left from the noted functions. $100J-(30J+50J)=20J$
Production efficiency = $\frac{Net \space secondary \space production\times100%}{Assimilation \space of primary \space production}$
Production efficiency = $\frac{20\times100}{30+20}$
Production efficiency = 40%
