This is a Rate problem.
First, we must establish the constant work required: (3 men)(2 hours) / 80 boxes
Don't compute it yet, just read "X manhours per box"
(note: you might want to compute boxes per manhour if you needed to
determine productivity output, but this problem asks for time (how long?)
so we do manhour/box)
Now, multiply by the number of boxes (boxes cancel out, leaving manhours):
(3 men)(2 hours)(150 boxes) / (80 boxes)
That's the manhours per box.
You want to use 5 men to make it faster, so divide by (5 men) and men cancels out leaving just hours.
(3 men)(2 hours)(150 boxes) / ((80 boxes)(5 men))
2 hours 15 minutes
Getting the units correct (and knowing when to multiply or divide) is the key to solving rate problems.