Settle a dispute?

[ubersonic]

I said 430...

My mate is saying 32.

He is saying because its ambiguous, he is reading it as half of 200, so he is doing

(200 / 100) + 30 = 32

You're both correct.

As he says, it's an ambiguous question with multiple valid answers (which was designed this way purely to create disagreements in Facebook comments).

 

[dowie]

it's an ambiguous question with multiple valid answers (which was designed this way purely to create disagreements in Facebook comments).

^^^ this

In the context of a casual request, it would be reasonable to assume it's sloppy English and the intent was to divide *in* half, otherwise why didn't they say "double" or "multiply by two".

It sounds like something some northern monkeys say:

"ey up lads, divide them bricks by 'alf"

Could be a request to divide the bricks into two equal piles, not to go out and get another load of bricks to double the size of the pile!

So that gives 130.

Of course strictly speaking, if treating it as a simple equation and if taken literally then the correct answer is the one the OP gave: 430

As for OP's friend, that seems to require an assumption "divide 200 by half of 200..." which is a bit odd, doesn't seem to follow.

As Dis86 said:

That's...not even an option

 

[dowie]

Half can be represented as 100/200 too. Not sure your point lol. Other than some unwritten rule where half is 0.5. Unless you are thinking of percentages in which case windows calculator 200 / 50% = 4.

Those are all the same quantity! It's not an unwritten rule, 100/200 literally equals 0.5 which also literally equals 50%

As for Windows calculator, can you not spot the obvious error here?

It's dividing by 50 there, so it's ignoring the % sign.

 

[dowie]

You swap, because you increase your probability from 1/3rd to 1/2 now.

When you first picked a cup, it was 1/3rd.

But if you pick again, it is now 50/50. If you stick with the original decision, you stick with the original probability.

This is incorrect.

Original odds were 1/3.

As a cup with no gold in was removed you have to recalculate the original odds since hes told a cup with no gold in was removed.

The chance he picked the correct one was ion fact 1/2 and is now still 1/2.

You cannot have odds less than 100/100.

You're both wrong.

Raymond was on the right track; the original choice was 1/3 and switching gives a better chance, but the probability of switching being correct is 2/3 not 1/2.

MKW - minor point but odds are not the same as probabilities. The second and more important point is that you don't have a time machine, the probability of picking the correct cup initially was 1/3 and that hasn't changed, you only have additional information about the unselected cups.

@Pants has correctly pointed out that this is a variant of the Monty Hall problem and provided a simulation. I guess there is an unwritten assumption here re the host offering the switch each time, that he isn't some evil host who only offers the switch when you've picked the prize etc.

But what might help this poster is another example where it's taken to an extreme:

I am told, with absolute certainty that mathematically i should swap cups to have the best chance of winning.. however i cannot get the reasoning why straight in my head.

Consider a deck of 52 cards laid out on the table face down, I offer a prize if you can pick the Ace of Diamonds say. I know where the Ace of Diamonds is.

You pick a card. I then flip over 50 other cards that I know are not the Ace of Diamonds.

So you're now left with the card you picked and one other card. You have an offer to switch.

What is the probability you picked the Ace of Diamonds out of 52 cards?

1/52

What is the probability that the other card is the Ace of Diamonds?

51/52

Should you switch? Yup!

(edit - FML I started saying odds later in the post too)