Question 781727
r*t=d for rate multiplied by time equals distance.

t=d/r and we have the d cut into {{{0.5d+0.5d}}}.


First half: {{{0.5d/20}}}, hours
Second half: {{{0.5d/15}}}, hours


The sum of those times must be 3.5 hours.
{{{highlight(0.5d/20+0.5d/15=3.5)}}}
{{{0.5d(1/20+1/15)=3.5}}}
{{{d(3/60+4/60)=3.5/0.5}}}
{{{d(7/60)=7}}}
{{{d(7/60)*(60/7)=7(60/7)}}}
As You Were Told as Key.