Question 1040382
Let {{{ d }}} = the original dose of medicine
{{{ 66 =  d*( 1 - .16 )^7 }}}
{{{ 66 = d*.84^7 }}}
{{{ 66 = .2951d }}}
{{{ d = 223.66 }}} mg original dose
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{{{ 223.66/4 = 223.66*.84^t }}} 
{{{ .84^t = 1/4 }}}
Take the log of both side
{{{ t*log(( .84 )) = log(( .25 )) }}}
{{{ -.07572t = -.6021 }}}
{{{ t = 7.9511 }}}
In 8 hrs less than 25% of the original dose will
be in the bloodstream
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You might want a 2nd opinion on these, also