Answer:
Step-by-step explanation:
If the side lengths are all different, none.
__
The following 7 combinations will form triangles:
(5, 5, 5), (5, 5, 7), (5, 7, 7), (5, 15, 15)
(7, 7, 7), (7, 15, 15)
(15, 15, 15)
_____
Additional comment
The triangle inequality requires the sum of the two shortest lengths exceed the longest. Here, we have 5+7=12<15, so a triangle with all three lengths cannot be formed. We also note that 5+5=10<15, and 7+7=14<15, so isosceles triangles with only one side equal to 15 cannot be formed, either.
