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?
There are 11 sets of tables and each set has 13 chairs. 

This means that there are 11 times 13 chairs in the backyard. 

11 times 13 is 143. 

So, there are 143 chairs in the backyard.