Question 476446
Let {{{ p(t) }}} = The number of participants, {{{p}}}, during year {{{t}}}
Let {{{ t = 0 }}} for the year 1970
then {{{ t = 30 }}} for the year 2000
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The equation is 
{{{ p = m*t + p(0) }}} ( p is in millions)
{{{ p(0) }}} is the count of participants for 1970 
{{{ p(0) = 22.4 }}}
The slope,m,  is (change in p) / (change in t)
{{{ m = ( 27.2 -22.4 ) / ( 30 - 0 ) }}}
{{{ m = 4.8 / 30 }}}
{{{ m = .16 }}}
The equation is:
{{{ p = .16t + 22.4 }}}
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In 2005:
{{{ p = .16*35 + 22.4 }}}
{{{ p = 5.6 + 22.4 }}}
{{{ p = 28 }}}
The estimate for 2005 is 28 million students