From the question statement, we know that the total amount of money that can be spent is $16 dollars and that the base cost of the party is $11 dollars, then we know that the money that the available money to spend would after paying that initial amount of money, by subtracting $11 from $16, like this:
16-11=5
Then we know that after spending the initial $11 we have $5 left, and we know that each guest costs 1$, which means that we can only invite 5 guests since 5 guests times $1 equals $5:
5*$1=$5
