Question 1039333
The sequence for the rows is:
{{{ 15 + 0*3 }}}
{{{ 15 + 1*3 }}}
{{{ 15 + 2*3 }}}
{{{ 15 + 3*3 }}}
{{{ 15 + 4*3 }}}
etc.
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If you have {{{ n }}} rows, the sum would be:
15*n + 3*( 1 + 2 + 3 + . . . + (n-1) )
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If {{{ n = 12 }}} and {{{ n - 1 = 11 }}}, you 
can use a formula for the sum of the digits:
{{{ 15*12 + ( 12*11 ) / 2 }}}
{{{ 180 + 132/2 }}}
{{{ 180 + 66 = 246 }}}
The maximum number of people who
can be seated is 246