Question 167202
d=r*t in this instance {{{r[d]=r[b]+r[c]}}} {{{r[u]=r[b]-r[c]}}} where {{{r[d]}}}rate downstream and {{{r[u]}}} rate upstream {{{r[b]}}}rate of the boat  {{{r[c]}}} is the rate of the current  {{{r[d]=45/5=9}}}


we know that {{{45=r[d](5)}}}---->{{{45=(r[b]+r[c])5}}}
:            {{{45=r[u](9)}}}---->{{{45=(r[b]-r[c])9}}}

{{{45=5r[b]+5r[c]}}}
{{{45=9r[b]-9r[c]}}}

multiply top by 9 and bottom by 5 to eliminate the {{{r[c]}}}terms

{{{90r[b]=630}}} {{{highlight(r[b]=7)}}}

so {{{highlight(r[c]=9-7=2)}}}