This question already has an answer here:
OOOOOOK! So. Here we go.
Your friend (a restaurant owner) calls you one afternoon in urgency. Something terrible has happened and he needs your support at once!! You borrow $50 from the neighbor's kid (as you are broke -as always-) and get a cab to his place. You see him sitting head-in-knees.
"What's up brah?" You ask.
"I'm ruined!" He replies. "A group of high dignitaries are coming to my restaurant and the special lamb-fried-rice I prepared for them is spiked with poison!"
You wonder for a moment why a group of high dignitaries would come to a useless place as this restaurant, but * ahem ahem *. So you go on to ask "Is all the dish spiked?"
"No. Only 1 plate. But demons take me, I don't know which plate it is!"
"Calm down man and tell me things from the start!" You tell him.
"OK here goes. A group of 16 dignitaries was coming to my place and they are all Asian. So I prepared our high specialty dish, lamb-fried-rice. Now it happens that one of my waiters was demanding me a salary increment for previous 4 months and I had been refusing him (oh you cheap skunk!). Now as a cook was passing around the kitchen area, he saw the waiter take out a small bottle from his pocket and add it to a plate (there were 16 plates on the row). He immediately went and confronted him about it, on which, that idiot jumped from the window of 3rd floor and ... died on the spot. In the meantime, other waiters had picked the plates up and left for placing them on the table. So now the idiot cook doesn't even know which plate it was that was poisoned.
"We have 4 poison testing machines with us which can detect the minutest amount of poison in given sample. But all machines can work on only one sample at a time and each machines take 55 minutes for producing the result. The dignitaries are arriving in 1 hours and 15 minutes. How the demons am I supposed to find the spiked plate in this time? Aaaaaaargh! I'm ruined. Ruined! Ruined! RUINED!!"
In Simple Words
There are 16 plates.
Only one plate is poisoned.
There are 4 poison testing machines.
Each machine can work on only one sample at a time. The result is delivered only as positive (sample contains poison) or negative (sample is not poisoned).
Each machine takes 55 minutes to process the sample.
The dignitaries are arriving in 1 hour and 15 minutes (75 minutes).
You must find the poisoned plate within this time.
NOTE: This is a puzzle which computer science students will be able to solve easily. Others will most probably not even get on the right thinking track. Not saying that it's impossible or something, but other people will probably make wrong assessments and start with wrong model in the very first place. THIS IS NOT A MATHEMATICAL PUZZLE
P.S. If you find the poisoned plate, don't expect your friend to pay you the 50$ you borrowed to reach him. If he was such a generous man, he would have increased the salary of the poor waiter!