Step 1:
Write the given data
Cost of renting hall = 300
Charge per guest = 65
Step 2:
Write an equation to represent the cost function
y = Total cost
x = number of guest
y = 300 + 65x
Step 3:
y = 300 + 65x
x = 75
Substitute x in the equation to find the total charge for 78 attendees.

Final answer
The total charge of 78 attendees = 5370