The pole, guy wire and ground forms a right angled triangle with the guy wire being the hypotenuse.
Legs = 10 ft and 15 ft.
The length of guys wire = Sqrt (10^2+15^2) =

≈18.03 ft
A total of 4 guy wires.
Te total length required = 4*

≈ 72.11 ft.
Since wire cannot be bought in fractions of foot, the length is rounded to the next whole number. That is,
Length required = 73 ft.