Kenny And The Toy Car

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September 5th 2007, 02:11 PM
Obsidantion

Kenny And The Toy Car

In 1 of every 5 bars of 'Choco Fudge', a chocolate bar costing 30p, is a coupon which can be traded in for a toy car at any shop which sells them. Kenny is set on having one of these toy cars but he doesn't have enough pocket money to buy any of the bars. His father says that if he can tell him how much it would cost to buy just enough bars for there to be at least a 90% chance of him getting a coupon, he'll give Kenny the money to buy the toy car. But he can't figure it out.

Help Kenny get his toy car by posting the answer in this thread!
September 6th 2007, 05:18 PM
fruitbodywash

he would need to buy 5 bars to get at least a 90% chance of getting the coupon. (because 4 only gets him an 80% chance) at 30p a bar, it would cost him 150p.
September 6th 2007, 10:17 PM
Obsidantion

Quote:

Originally Posted by fruitbodywash

he would need to buy 5 bars to get at least a 90% chance of getting the coupon. (because 4 only gets him an 80% chance) at 30p a bar, it would cost him 150p.

I can see what you've done here, it's not correct I'm afraid but I'll explain to you why:
When an event has a certain chance of an outcome, say flipping a coin and the 50% chance of getting a heads, if you flip the coin twice, the chance of you getting a heads in either of those flips is not the chance of getting a heads in 1 flip 'plus' the chance of getting a heads in another. If it were, that would mean there would be a 100% (50% + 50%) chance of getting a heads when you flip a coin twice, but we all know that you could flip 2 tails, so 100% is not right. What you've done is you've recognized that there is a 20% chance of getting a toy car in 1 chocolate bar and you’ve multiplied that by the number of chocolate bars you buy which isn't correct because then there would be 100% chance of getting a toy car when you buy 5 bars, but it’s possible that every bar could not contain a car.

The best way to solve this problem for you would be to consider simpler examples of probability (like the chance of getting a heads in 2 coin tosses) and see what you discover, then apply your findings to this problem.