Answer:
Let x be the number of person and y be the total cost per person.
As per the statement:
A catering company charges a $300 set up fee and $10 per person for a lunch buffet.
"$10 per person" translated to 10x
then;
Total cost per person = 10x+300
It is also given that: total cost per person is $14.
⇒

Subtract 10x from both sides we have;

Divide both sides by 4 we have;
x = 75
Therefore, 75 number of people must attend the lunch so that the total cost per person is $14