read here for chapter 15 solution

Kick off the outcrop to the lighting rig above, follow it over to the
left. Two security guards will find the stashed laptop and will begin
heading over the bridge to the Skobel tower.

Drop down and follow them. Jump over (Triangle Button) the Perspex
barrier beside the turnstiles. Funnily enough, the other side of the
bridge is your first checkpoint... The guards will turn right, but
going that way will only lead to trouble, so go left.

There are two guards in the hall ahead, thankfully they are facing
away from you. So head through the doorway halfway down the hall, on
right, to enter some offices. Use Creep Mode (Circle Button) and
creep past the two offices workers on your left... haha... lesbians.

Open the door (Triangle Button) on your right just past the workers,
it leads to the main antrum. Head into the kitchen area on your
right and then to the hallway on the right again. A single guard
is up ahead, thankfully facing away, go left before him into a
waiting area with TVs. Head right along the hallway ahead (because
there is a lone guard facing away to your left) and enter more
offices via the door on your left at the end of the hallway.

You have reached the second checkpoint. Open the door ahead, there
are two office workers in here (a man working and a woman supervising
him). To evade them, crouch (X Button) in front of the desk beside
the man and then crawl under the desk (Square Button). Crawl to the
other side and then press the Square Button once more to scarper out
of there. You can head out to the hallway now, because the woman
has gone to her desk.

In the hallway, to your right are two guards and on your left is a
lone guard. Kick off an outcrop to the lighting rig and follow it as
far as you can to the left (past the lone guard) and then drop down.
You will see the two guards with the laptop, creep up behind them to
the 13th Floor (IT Department).

They will head right to this floors antrum, but it is guarded, so
you must head down the hall directly ahead. But before you
do, head through the red porthole door on your left, this chapter's
key and a MedKit are inside. Anyway, follow the hallway, two
Russians are up ahead. So head into the Coms Server Room on your
right, halfway down the hall.

Sneak left past the technician, flatten against the wall (X Button
when not holding the Left Analogue stick) and shimmy past the servers
to the other side. Go through the nearby door. Head right down the
hall and then left into this floor's kitchen, up ahead (across the
hall) is the door leading to the server room, enter it. You have
reached the third checkpoint.

The guards with the laptop are in the room up ahead; follow them up
to the 15th Floor (Investment). You have reached the 4th checkpoint.
Follow the hallway and soon you will see a short cut-scene hinting at
more rig crawling. So climb up and crawl to the right to follow the
guards, they will walk along the adjacent offices, follow the rig all
the way to the end and then wait for the guards. Once they have
walked past, drop down and follow them to the stairway. A cut-scene
will play to avoid the climb to the 20th Floor (Executive Suit).
Just follow them to Victor's Office to retrieve the laptop.
:) ;) :D ;D ::) :P