The additional number of people is [4, 13]
Brittany and four of her friends are five people.
So, we have:
n = 5
The conditions are given as
Let the additional number of people be x
So, we have the following inequalities
x + 5 ≥ 9 and x + 5 ≤ 18
Subtract 5 from both sides
x ≥ 4 and x ≤ 13
Express as interval notation
[4, 13]
Hence, the additional number of people is [4, 13]
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