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# Very Quick S1 Question watch

1. Why does P(1.3≤X1 + X2≤3.2) round to p(2≤X1+X2<3) surely it would be P(1≤X1+X2≤3)
2. (Original post by creativebuzz)
Why does P(1.3≤X1 + X2≤3.2) round to p(2≤X1+X2<3) surely it would be P(1≤X1+X2≤3)
Well, if X1 and X2 are discrete random variables...
3. (Original post by BuryMathsTutor)
Well, if X1 and X2 are discrete random variables...

Erm, could you elaborate on that please?
4. (Original post by creativebuzz)
Erm, could you elaborate on that please?
Could I see the whole question please?
5. Attachment 461381461383
(Original post by BuryMathsTutor)
Could I see the whole question please?
Oh yes of course, sorry! (two attachments)
Attached Images

6. (Original post by creativebuzz)
Attachment 461381461383

Oh yes of course, sorry! (two attachments)
As I expected, X is a discrete random variable. This means, in this case, X takes only certain integer values. X1+X2 is also an integer. If for example then in fact .
7. (Original post by BuryMathsTutor)
As I expected, X is a discrete random variable. This means, in this case, X takes only certain integer values. X1+X2 is also an integer. If for example then in fact .
Do you take all 3 maths exams in the summer??
8. (Original post by BuryMathsTutor)
As I expected, X is a discrete random variable. This means, in this case, X takes only certain integer values. X1+X2 is also an integer. If for example then in fact .

But why is 1.3 rounded UP to 2 yet 3.2 is rounded DOWN to 3?
9. (Original post by creativebuzz)
But why is 1.3 rounded UP to 2 yet 3.2 is rounded DOWN to 3?
If then and so
10. (Original post by BuryMathsTutor)
If then and so
I just don't understand why we are rounding up for 1.3 but rounding down for 3.2?
11. (Original post by creativebuzz)
I just don't understand why we are rounding up for 1.3 but rounding down for 3.2?
Think about what values X1 + X2 is allowed to take!

If X1 + X2 must be a whole number, then if X1 + X2 >= 1.3 we must have X1 + X2 >= 2. It isn't possible for it to be as small as 1.

If X1 + X2 <= 3.2 then we know that X1 + X2 <= 3. It isn't possible for it to be as big as 4.

It's the fact that X1 + X2 must be an integer that is crucial here.
12. (Original post by creativebuzz)
I just don't understand why we are rounding up for 1.3 but rounding down for 3.2?
It's because X1 + X2 has to be greater than 1.3, but it's gotta be less than 3.2. If you rounded 1.3 down, you'd be including values between 1 and 1.3, which we can't include as they don't fit within our inequalities. So we round up, because everything greater than 2 is also greater than 1.3. Similarly, we can't round 3.2 up cause then you'd include values between 3.2 and 4, which don't satisfy our inequality. So we round down, because everything less than 3 is also less than 3.2.

Hope that makes sense.
13. (Original post by davros)
Think about what values X1 + X2 is allowed to take!

If X1 + X2 must be a whole number, then if X1 + X2 >= 1.3 we must have X1 + X2 >= 2. It isn't possible for it to be as small as 1.

If X1 + X2 <= 3.2 then we know that X1 + X2 <= 3. It isn't possible for it to be as big as 4.

It's the fact that X1 + X2 must be an integer that is crucial here.
Got it now, thank you!

I know how to do this question now but my first attempt was to do:

Not winning first:
=1-0.5
=0.5

Not winning 2nd:
1-0.3
=0.7

Therefore not winning first and second is 0.7 x 0.5 = 0.35

Even though that answer is wrong, can you explain why my method was incorrect? (Just so I don't make the same mistake in the future)
14. (Original post by creativebuzz)
Got it now, thank you!

I know how to do this question now but my first attempt was to do:

Not winning first:
=1-0.5
=0.5

Not winning 2nd:
1-0.3
=0.7

Therefore not winning first and second is 0.7 x 0.5 = 0.35

Even though that answer is wrong, can you explain why my method was incorrect? (Just so I don't make the same mistake in the future)
Can you post the actual question so I can check the wording? This doesn't seem to relate to the question you've already posted in this thread

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