Question 607052
The rows are:
{{{ 10 + 0 }}}
{{{ 10 + 1 }}}
{{{ 10 + 2 }}}
{{{ 10 + 3 }}}
All the way to {{{ 40 }}} rows, so
This is the same as:
{{{ 10*40 }}} + 0 + 1 + 2 + 3 + ... + 39
This is {{{ 400 }}} + (1/2) x (total number of rows) x 39
{{{ 400 + (1/2)*40*39 }}}
{{{ 400 + 780 = 1180 }}}
There are 1180 seats