The whiteboards she bought cost $35, and she spent a total of $51.20, subtract to find out the total amount she spent on posters:
[tex]\sf 51.20-35=16.2[/tex]
So she spent a total of $16.20 on posters, now divide by the amount one poster costs to find out how many posters she bought.
[tex]\sf 16.2\div 5.4=\boxed{\sf 3}[/tex]
So she bought 3 posters.
