Book Maths Puzzle Averages

A student has grades on three examinations of 76, 80, and 72. What must the student earn on a fourth examination in order to have an average of at least 80?

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Book Other Question

this is an Ellipses problem but I can't figure it out. Could you please help me.

x^2+4y^2-10x-40y+121=0
The perimeter of a square is to be between 17 and 56 feet. Find all

possible values for the length of its sides.