let the total food for the eaten is $$ 1 $$ and out of these $$ \dfrac {1}{10} $$ of the food eaten is turned into organism own body.
Now available for the next level of the consumer in a food chain is $$ \Rightarrow 1 - \dfrac {1}{10} = \dfrac { 10-1}{10} = \dfrac {9}{10} $$