Step 1: A and B cross the bridge with the lamp.(2 mins) Step 2: A comes back with the lamp.(1 mins) Step 3: C and D cross the bridge with the lamp .(11 mins) Step 4: B brings back the lamp.(2 mins), Step 5: A and B cross the bridge with lamp.(2 mins) Total minimum time required =2+1+11+2+2=18mins.