Three or four.

Let us use three-sided pyramids (sometimes called tetrahedrons). To make the pyramid, start with a triangular arrangement of tennis balls for the bottom layer. Next stack up tennis balls layer at a time, as at right.

If the base of the pyramid has 5 balls along a side,
how many balls does the entire pyramid have in it?
Let us call this a **pyramidal number**.

Here is a chart that shows the number of balls in the side of a base,
**N**,
and the corresponding pyramidal numbers.

N | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
---|---|---|---|---|---|---|---|

Pyramid(N) | 1 |