Stefan requires 6 hours to wash the cars by himself. In one hour he washes 1/6 of the cars. Misha can wash the cars by herself in 5 hours. In 1 hour she can wash 1/5 of the cars.
Stefan has 2.5 hours to wash the cars as he has to go to the football game. In 2.5 hours he has washed (1/6)*2.5 = 5/12 of the cars. This leaves 1 - 5/12 = 7/12 of the cars for Misha to wash. As she washes cars at the rate of 1/5 per hour, it would take her (7/12)/` `(1/5) = (5*7)/12 = 35/12 hours to wash the remaining cars.
35/12 hours is equal to 175 minutes or 2 hours 55 minutes.
Misha finishes washing the cars in 35/12 hours after Stefan leaves.