Question 1127041
downstream, {{{(20+5)*x=1}}}
{{{x=1/25}}}


upstream, {{{(20-5)*y=1}}}
{{{y=1/15}}}


average speed for the whole round-trip,
{{{(1+1)/(1/25+1/15)}}}


-


{{{2/(1/25+1/15)}}}


{{{(2/(1/25+1/15))((5*5*3)/(5*5*3))}}}


{{{(2*5*5*3)/(3+5)}}}


{{{(2*5*5*3)/(2*4)}}}


{{{(5*5*3)/4}}}


{{{75/4}}}


{{{highlight(18&3/4)}}}, {{{km/hr}}}