Scroll down for a solution to this problem.
The total length is 120/13, which is approximately 9,23.
Next, acknowledge that triangle ABC is the union of triangles ADC and BDC, the heights of which are a and b respectively and which both have base 13. Their total area is (a+b)*13/2. Equating this with 60 gives the desired result.
Skip stones in a triangle
Are playing and jumping
From one side to the other
From one angle to one corner
Flat stones into the river throwing
The pebble is surfing
On the water stone skimming
A wave ballet dancing
Wonderfull nature is doing