You have 6 sticks of lengths 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 from the following answers:  A) 3  B) 6  C) 7  D) 13  E) 12
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.
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You have 6 sticks of lengths 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