Find the Shortest time ?

Four boats A-D are on the left bank of a river.
To Cross the river to the right bank,
Boat A takes 2 minutes;
Boat B takes 4 minutes;
Boat C takes 8 minutes; and
Boat D takes 16 minutes.
The four boats need to go the right bank, but there is only one boatman.
One boat can pull another boat, but it will take the same amount of time as the slower of the two boats to cross the river.
If the boatman uses one boat to pull another boat to the right bank and returns to the left bank in one boat,
How many minutes will it take for him to move all the boats to the right bank?
Find the shortest time ?
(Ignore the time he spends changing and connecting boats)

Click here for Answer

The Shortest time is 30 minutes.
The logic is as follows :
A and B takes 4 minutes
Return in B another 4 minutes
C and D takes 16 minutes
Return in A takes 2 minutes
Now take A and B takes 4 minutes
So total time taken is : 4+4+16+2+4 = 30 minutes