Whats 20% Of 687 If The Sum Of The Numbers Is 8 And The Sum Of Their Cubes Is 20 Whats Is The Sum Of Their Squares?

If the sum of the numbers is 8 and the sum of their cubes is 20 whats is the sum of their squares? - whats 20% of 687

Please help me, because the morning and took me all day and I couldnot resolve this issue .. Thank u ,,,,,,