20. Bernard, Ai Ji, Yidan and Laurie, four members of U2 Chorus, rushed to the performance site. On the way, they had to cross a small bridge. When they reached the bridge, it was already dark and there were no lights around. At most, two people cross the bridge together at a time. People crossing the bridge must have flashlights in their hands, and flashlights can't pass in the same way. Four people walk at different speeds. Two people walk together, whichever is slower. Bernard needs 2 minutes to cross the bridge, Ai Ji needs 3 minutes, Edam needs 5 minutes, and Laurie needs 10 minutes. Excuse me: What is the shortest time = _ _ _ _ _ _ _ _ _ _ _.

Obviously, Bernard spends the least time with a flashlight, and the gap between getting and working too long is smaller than that of Bo, so

3 (Bo 2, ai 3)+2 (bo 2)+10 (Lao10, get 5)+3 (Ai 3, Bo 2)=2 1 min.

So 2 1 at least yo, you can ask me if you don't understand, and hope to adopt it.