Final answer:
To find the maximum number of people they can invite, solve the equation $90 + $30x ≤ $2,500, where x represents the number of people.
Step-by-step explanation:
To find the greatest number of people that Mark and Heather can invite and still stay within their budget, we need to set up an equation based on the given information.
The equation is: $90 + $30x ≤ $2,500, where x represents the number of people they can invite.
By solving this equation, we can determine the value of x and find the maximum number of people they can invite.