To determine how many additional people attended the birthday party, we can use the given linear cost function \( b(x) = 100 + 15x \), where:
- \( b(x) \) is the total cost of the party,
- \( 100 \) is the rental fee for the space,
- \( 15 \) is the cost per person attending,
- \( x \) is the number of additional attendees (excluding Leslie and Steve).
We know that Leslie received a bill of $550:
\[ b(x) = 550 \]
Now, we can set up the equation:
\[ 100 + 15x = 550 \]
Next, we'll solve for \( x \):
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Subtract 100 from both sides: \[ 15x = 550 - 100 \] \[ 15x = 450 \]
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Divide both sides by 15: \[ x = \frac{450}{15} \] \[ x = 30 \]
Since Leslie and Steve are already counted, the total number of people that attended the party is \( x + 2 \) (the two of them), which is:
\[ 30 + 2 = 32 \]
Thus, the number of other attendees (excluding Leslie and Steve) is 30.
Therefore, the answer is 30.