Question 608972
Let {{{ n }}} = the number of 20 year periods
since 2000
{{{ 100 = 6.1*2^n }}} ( answer in billions )
{{{ 2^n = 100/6.1 }}}
{{{ 2^n = 16.3934 }}}
{{{ n*log(2) = log( 16.3934) }}}
{{{ .30103n = 1.21467 }}}
{{{ n = 4.035 }}}
{{{ 20n = 20*4.035 }}}
{{{ 20n = 80.7 }}}
and {{{ 2000 + 80 = 2080 }}}
In 2080 the world population will reach 100 billion
Here's the plot:
{{{ graph( 400, 400, -1, 6, -10, 120, 6.1*2^x ) }}}
The horizontal axis is number of 20 yr periods
The  vertical axis is population in billions