Milana has had six tests in her math class so far. She knows the average (mean) of some of her marks.
The average of her first and second test marks is \(85\%\).
The average of her second and third test marks is \(82\%\).
The average of her third and fourth test marks is \(74\%\).
The average of her fourth and fifth test marks is \(79\%\).
The average of her fifth and sixth test marks is \(80\%\).
Determine the average of her first and sixth test marks.