Question 1183833
Whole trip, d
Up the mountain, d/2
Down the mountain, d/2


Time going up, {{{(d/2)(1/5)}}}
Time going down, {{{(d/2)(1/7.5)}}}
Whole time, {{{d/10+d/15=3d/30+2d/30=5d/30=d/6}}}


Average speed for whole trip
{{{d/(d/6)}}}

{{{highlight(6)}}}kilometers per hour