(time in hrs it normally takes) + (2 hrs + 40 min) = (longer time when he runs slower)

clear the decimal in the last fraction ...

common denominator is ...

numerators form the equation ...

only solution which works in the context of the problem is v = 10 km/hr

at 10 km/hr, takes 8 hrs to run 80 km

at 7.5 km/hr, takes 10 hrs 40 min