In 2/3 of a hour, workers had installed 2/5 of a foor. At the rate, what fraction of the flooring is finished after 1 hour?

3/5 or 60%.

2/3 hour is 40 minutes, so 1/3 is 20 minutes.

40 minutes have passed and 2/5 of the progress on the floor is complete. In 20 minutes, 1/5 of the work is complete. Add 20 minutes to 40 minutes to get 60 minutes, or an hour.

Because of this, we need to add the progress of 20 minutes and 40 minutes. 1/5+2/5=3/5.