Hilarious. Looks like the education system in the UK sorely needs to be revamped.

Um, okay. I think I can see it now. Not sure of the practical applications.

Let's just say I didn't do very well in calculus.

alevel qn for 18 y/o? try this then...

What's the answer?

What's so difficult about it?

996-999
987-989
978-979
969

897-899
888-889
879

798-799
789

699

-notice the pattern-

(4+3+2+1+3+2+1+2+1+1)/1000 = 20/1000 = 0.02

-------------------

999
888

(1+1)/1000 = 2/1000 = .002

(a) 1/50 = 0.02

(b) 1/10 = 0.1

I don't get it. What pattern Doc? Where did you get the 1/50 from sgcat? Explain step by step.

You have the digits 0-9 to work with. Any combination of three has to equal 24. 24 is a big number, so it makes sense to use as many 9's as possible.

Thus. 24 - 9 = 15. 15 - 9 = 6.
So the first number you start with is 996. Obviously is 9+9+6 = 24, then any 3 digit number greater than that will sum over 24. So your first set of solutions is: 996-999

Now let's go back. 24 - 9 = 15. Subtract 8 this time, and you get 7.
987. Any 3 digit number greater than that but less than 990 will sum greater than 24. So: 987-989

So you keep doing this process until you get all of the combinations.

But the pattern is this: Notice that for the first number in the set, the 10s place goes down by one and the 1s place goes up by one:

996-999
987-989
978-979
969

For the second number, the 10s place goes down by one:

996-999
987-989
978-979
969

So just apply that simple pattern over and over again until it stops working and you get this:

996-999
987-989
978-979
969

897-899
888-889
879

798-799
789

699

If you count up the numbers per set and add them, it looks like this: (4+3+2+1) + (3+2+1) + (2+1) + (1). Add that together and you get 20. Divide by 1000 (000 - 999 is 1000 numbers). You get .02

For the second half, you just look at the sets you got and find the numbers that have identical digits. The onlys ones are 999 and 888. So that's 2/1000 which is .002

Oh wait, that's pretty cool.

Y'all smarter than I expected (no shade)

I think I used to be smarter I should do something about that.

the answer for the 2nd part is 0.1 as it is a conditional probability. but .002 is correct if we ignore the 'given that' part

It isn't that difficult cmon.
When I did my GCSE's we thought it was the worst thing too. Wait until they've done advanced maths at university

Mess, I read it as "are identical and sum to at least 24". Let me revise:

Out of the 20 numbers that sum to at least 24, two of them have identical digits. 2/20 = 0.1

mess is there any easier/ simpler way to get the answer to the question?
I swear I hate probability

did you learn permutations and combinations? its much easier that way

you're very good at this!

This thread makes me uncomfortable I've literally forgotten all maths since leaving school

I did those and I remember I hated them with passion

i remember during a-levels when everyone whinged about a difficult question on a maths paper. people forget that this **** gets scaled, so if everyone finds it difficult then everyone will be okay. they just have these harder questions to pick out the students who can apply knowledge rather than just follow routine.

This isn't hard but it is for a GCSE maths paper, I took the exam when I was 15 and if I had encountered this I wouldn't have been able to do it instantly I don't think, whereas the rest of the paper was just obvious IMO.