Here, A takes the shortest time to cross the bridgei.e. 1 min. And, D takes 10‌min,C takes 7‌min and B takes 2‌min to cross the bridge. So, 4 friends can cross the bridge in minimum time in the following ways. 1. A+B crossing and A returning :(2+1)‌min 2. A+C crossing and A returning : (7+1)‌min 3. A+D crossing and A returning :(10+1)‌min 4. A crossing the bridge for the last time: 1 min. Hence, total minimum time =3+8+11+1=23min
