stefan and misha have a car washing business. Stefan could wash the cars by himself in 6 hours and misha could wash the cars by himself in 5 hours. stefan starts washing the cars by himself, but he needs to go to his football game after 2.5 hours. misha continues the task. how long did it take misha to finish washing the cars?

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 2 hours and 55 minutes after Stefan leaves.