You have 7 sticks of lengths 10, 20, 30, 40, 50, 60 and 70 centimeters. The number of non-congruent triangles that can be formed by choosing three of the sticks to make the sides is Choose the correct choice
A) 3
B) 6
C) 7
D) 10
E) 13
The length of any side of a triangle must be larger than the positive difference of the other two sides, but smaller than the sum of the other two sides.   Based on this there can be only 13 triangles formed: (20, 30, 40), (20, 40, 50), (20, 50, 60), (30, 40, 50), (30, 40, 60), (30, 50, 60), (40, 50, 60), (50, 60, 70), (40,60, 70), (30,60, 70), (20,60, 70). (30, 50, 70), (40, 50,70)   Answer; 13.
<result>E</result>
You have 7 sticks of lengths 10, 20, 30, 40, 50, 60 and 70 centimeters. The number of non-congruent triangles that can be formed by choosing three of the sticks to make the sides is