If a student draws the first four triangular number dots on board. Each number represents an equal number of dots. Then find the mean of these dots.

Option 2 : 5

**Given: **

The first four triangular numbers are 1, 3, 6, 10.

**Formula used:**

Triangular number = n(n + 1)/2, where n = 1, 2, 3, 4………

\({\rm{Mean}} = {\rm{\;}}\frac{{{\rm{Sum\;of\;observations}}}}{{{\rm{Total\;number\;of\;observations}}}}\)

**Calculation:**

The first four triangular numbers are 1, 3, 6, 10.

\({\rm{Mean}} = {\rm{\;}}\frac{{{\rm{Sum\;of\;all\;triangular\;number\;dots}}}}{{{\rm{Total\;triangular\;number}}}} = \;\frac{{1 + 3 + 6 + 10}}{4} = \frac{{20}}{4} = 5\)

**∴ The mean of the first four triangular dots is 5.**