Magic Triangle
Place the digits 1 to 9 into the circles so that each side of the triangle adds to the same total.
Reference 1 (introduction): Learn-with-Math-Games
Reference 2 (demonstration): Math Dad vs Science Mom
Reference 3 (mathematics):
1. Perimeter Magic Triangles
http://www.magic-squares.net/perimeter.htm
2. Theorem concerning a Magic Triangle - Teachers of India
http://teachersofindia.org/en/download/file/fid/19495