Question 1035430
Hi there,
If you connect the radius from
each of the points of the triangle
to the center of the pool bisecting
the 60 degree angles , you form 
3 isosceles triangles.
The angle at the center is 120 degrees
with 30 degrees at the two equal angles.
Using A,B and C to denote the points of 
one of the isosceles triangles, you have:-
Cos A = 120 degrees.
AB = 5 m
AC = 5 m
BC^2 = AB^2 + AC^2 - 2(AB) x (AC) x Cos(A)
BC^2 = 5^2 + 5^2 - 2(5) x (5)x cos(120)
BC^2 = 75
BC = π75
BC = 8.7 meters
3 x 8.7 = 26.1 meters
The total length of planks required.
Hope this helps :-)