Answer
$4H_{3}PO_{3} →3H_{3}PO_{4} + PH_{3}$
Work Step by Step
When $4H_{3}PO_{3}$ is heated, it forms $4H_{3}PO_{4}$ and $PH_{3}$.
Note that the same number of each of the elements must exist on the products and reactants side for each balanced reaction.
