Question 785369
{{{x}}}= time stopped at each port (in minutes)
{{{3x}}}= total time stopped at one or another port (in minutes)
The hydrofoil is moving for only 15 minutes of the trip (stopped for the rest of the trip time).
{{{3x+15}}}= total trip time (in minutes)
{{{(3x+15)/60=3x/60+15/60=0.05x+0.25}}}= total trip time (in hours)
The average speed is the total distance divided by the total time
{{{"50 km/h"=30km/(0.05x+0.25)h}}}-->{{{0.05x+0.25=30/50}}}-->{{{0.05x+0.25=0.60}}}-->{{{0.05x=0.60-0.25}}}--> {{{0.05x=0.35}}}-->{{{x=0.35/0.05}}}--> {{{highlight(x=7)}}}