A man is trapped in a computer or Virtual Reality experiment. The people running the experiment feed clues into the simulation to let the man know he is trapped and to give him information on how to get out.
NOT like the story 'Outside'.
This may have been Sci-Fi for young people from the 1960s.
I offer another possible answer: Time out of joint, also by Philip K. Dick (1959), in which a man is living in an artificial reality. He gets some clues about his real status, but they are not provided in order for him to escape; on the contrary, they are caused by imperfections in the virtual reality. And it's definitely not a story for young people.
I'm wondering if this is Ubik, by Philip K Dick. It's 1969, but not for young people (though I read it as a teenager). It's certainly about artificial reality and feeding clues to someone trapped inside. I believe there's a description of a man on a slab.
From Goodreads:
Glen Runciter runs a lucrative business—deploying his teams of anti-psychics to corporate clients who want privacy and security from psychic spies. But when he and his top team are ambushed by a rival, he is gravely injured and placed in “half-life,” a dreamlike state of suspended animation. Soon, though, the surviving members of the team begin experiencing some strange phenomena, such as Runciter’s face appearing on coins and the world seeming to move backward in time. As consumables deteriorate and technology gets ever more primitive, the group needs to find out what is causing the shifts and what a mysterious product called Ubik has to do with it all.
I have not read the book but I did some searching and it seems you may be referring to a short story from Stanislaw Lem possibly named "IJON TICHY'S MEMORIES" (1960).
From StrangePegs:
Remember Stanislaw Lem (from this post)? Well, in 1960, he had a short story, "IJON TICHY'S MEMORIES," in which a scientist creates an entire virtual world (and, evidently, traps people within it). Trapped in this virtual world is another scientist who creates a virtual world. Yes, within the first one.
Could it be Simulacron-3 by Daniel F. Galouye from 1964?
In the main character's world they have begun to create simulated worlds of their own, gradually various suspicious events drive him to the conclusion that his own world is a simulation too. It turns out that his appearance and personality are actually based on those of the "real" person running the simulation, the Operator, and the Operator isn't willing to just reprogram his memories or have him killed, but instead enjoys playing with him...
Meanwhile the Operator's lover is falling in love with the simulated version (the 'real' version has apparently become mad with power, so the simulated version reminds her of the man she originally fell in love with) and is trying to help him. Here's what she says about why the Operator let him live:
"It wasn't until yesterday," she went on, "that I realized he could have solved his problem, as far as you were concerned, any time he wanted, simply by reorienting you. But no. There was too much perverted gratification to be had by letting you come close to Fuller's secret, then pushing you away, steering you all the while toward some such fate as he arranged for Collingsworth."
Eventually the simulated version is somehow able to overwrite the personality of the version in the "real" world, and wakes up there. You mentioned that the simulation is in realtime, that's true in this story:
"Even the Operator can't stay at it twenty-four hours a day. This world is on a time-equivalent basis with the real one."
And you mentioned a scene where he's on a "slab" to interface with the virtual reality, here's the scene where he wakes up in the real world, though it sounds like more of a couch:
Confounded, I opened my eyes and was instantly confronted with the effects of a strange room spread out all about me. Although it was a room I had never seen before, I could recognize the simulectronic nature of the equipment that filled almost all available space. I glanced down and saw that I lay on a couch much like the one I had used before while coupled with reactional units in Fuller's simulator. I reached up and removed the empathy helmet, then sat staring incomprehensively at it. There was a couch next to mine. Its leather surface still bore the indentation of the person who had occupied it -- for a long while, judging from the depth of the impression. On the floor nearby were the shattered remains of another headpiece that had evidently been dropped or hurled aside.
If this isn't the one, there are a number of other early virtual reality stories mentioned at the Encyclopedia of Science Fiction.
“ I Have No Mouth and I Must Scream” Harlan Ellison 1967
More so about a group of people (with one man as the narrator), trapped in a computer called “AM” (I believe) that tortured them.
