# How to check percentage yields in a given reaction?

For a general reaction , if it is asked to predict and not given by theoretically, how would we determine the percentage yield of the products based on their mechanism , stability , path taken for the reaction etc. ?
For example , where my main problem lies is :

According to Solomon and Frhyle it is said that chlorine is highly unselective. But in a problem I was asked percentage yields of the two products:

So how do I go about determining the percentage yields in this reaction ?

What the text is trying to explain to you is that when comparing analogous reactions of $\ce{Cl2}$ and $\ce{Br2}$, there will be higher percentage yields of the more highly substituted (and stable) product in the $\ce{Br2}$ reactions. For example, in the reaction between propane and $\ce{Cl2}$, the yields of 1-chloropropane and 2-chloropropane are 45% and 55% respectively. In a reaction between propane and $\ce{Br2}$, the yields of 1-bromopropane and 2-bromopropane are 3% and 97% respectively.