Solution:

Let's say A takes 1 minute to cross, B takes 2, C takes 5, and D takes 10. To arrive at the solution, you can assume the following:

1) C and D are the slowest. Therefore, you want them to cross together, and not have either one carry the torch back to the other side. A round trip for C would take 10 minutes, and for D it would be 20 minutes!

2) A and B are the fastest. Therefore, we want to use them as the "couriers" who bring the lantern back.

	elapsed	total
Send A+B across first.	2 minutes	2 minutes
Send A back with torch	1 minute	3 minutes
Send C+D across	10 minutes	13 minutes
Send B back with torch	2 minutes	15 minutes
Send A and B over	2 minutes	17 minutes