They decided to hold the party in their backyard. If they have 11 sets of tables and each set has 13 chairs how many chairs do they have in the backyard?


Let $x$ be the number of chairs in the backyard. 
Then $x + 1$ is the number of chairs in the front yard. 
The total number of chairs is $x + x + 1 = 2x + 1$. 

Now we have the equation $2x + 1 = 24 \Rightarrow 2x = 23 \Rightarrow x = 11.5$. 

There are 11.5 chairs in the backyard.