One side of the bottom layer of a triangular pyramid has 12 balls. How many are there in the whole pyramid?
Note that the pyramid is equilateral and solid.

• Bottom layer contains (12+11+10+...+1) = 12*13/2 = 78 balls.
  Immediate top layer contains (11+10+...+1) = 66 balls.
  Similarly, the number of balls in the layers from top to bottom are 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78. Total = 1+3+6+10+15+21+28+36+45+55+66+78 = 364 balls
• 15 years agoHelpfull: Yes(6) No(0)
• 364
• 15 years agoHelpfull: Yes(3) No(1)