This is an equilateral triangle of height a√3/2 where a is the side length. So the area is ½a.a√3/2=a²√3/4. So a²=25 and a=5cm. Therefore the perimeter is 3a=15cm.
I guess it could represent a situation where you have of a cake left and you have to share it with three of your friends (in addition to yourself). So that's four people sharing of a cake.
