S3 Discrete Random Variable.

singh224

8

Hi can someone please guide me through part (b) of this question:

I can't seem to understand how to do it? :redface:

Thanks

Reply 1

Smaug123

15

Original post by singh224

I can't seem to understand how to do it? :redface:

It's not a matter of understanding how to do it, but of writing things down and seeing what happens. Under what conditions would the total length of material be 120m?

Reply 2

singh224

OP

8

Original post by Smaug123

It's not a matter of understanding how to do it, but of writing things down and seeing what happens. Under what conditions would the total length of material be 120m?

When there is order of 70m and 50m. Which is exactly 2 order but If I add the probability of those 2 I get 0.54. Is this right?

Reply 3

Smaug123

15

Original post by singh224

When there is order of 70m and 50m. Which is exactly 2 order but If I add the probability of those 2 I get 0.54. Is this right?

You've missed a possibility.
Also, by your logic, if I made three orders, there would be a 150% chance of getting 150m of material, so something's up with your method.

Reply 4

singh224

OP

8

Original post by Smaug123

You've missed a possibility.
Also, by your logic, if I made three orders, there would be a 150% chance of getting 150m of material, so something's up with your method.

Please can you tell me what the right method is? I'm self teaching S2 so I can't see where I'm going wrong as the book doesn't show examples like these.

Reply 5

ExWunderkind

12

Original post by singh224

Please can you tell me what the right method is? I'm self teaching S2 so I can't see where I'm going wrong as the book doesn't show examples like these.

You've missed the possibility of 60 and 60.

And you've added together your probabilities, instead of multiplying them.

Reply 6

singh224

OP

8

Original post by ExWunderkind

You've missed the possibility of 60 and 60.

And you've added together your probabilities, instead of multiplying them.

Thanks, so is the answer 0.0264?

Reply 7

ExWunderkind

12

Original post by singh224

Thanks, so is the answer 0.0264?

That's what I get, yes.

Reply 8

singh224

OP

8

Original post by ExWunderkind

That's what I get, yes.

Thanks, the book is wrong I guess? it says 0.0464 :/

Reply 9

Smaug123

15

Original post by singh224

Thanks, so is the answer 0.0264?

Yep, that looks about right to me.

Reply 10

ExWunderkind

12

Original post by singh224

Thanks, the book is wrong I guess? it says 0.0464 :/

I hate it when that happens, it does make you second guess yourself. But I would say it's a typo, unless I too am making a mistake.

0.50 \times 0.04 = 0.02

0.08 \times 0.08 = 6.4 \times 10^{-3}

0.02 + 6.4 \times 10^{-3} = 0.0264

To get the answer the book has you'd have to use 60 and 50 which =/= 120.

(edited 10 years ago)

Reply 11

singh224

OP

8

Original post by ExWunderkind

I hate it when that happens, it does make you second guess yourself. But I would say it's a typo, unless I too am making a mistake.

0.50 \times 0.04 = 0.02

0.08 \times 0.08 = 6.4 \times 10^{-3}

0.02 + 6.4 \times 10^{-3} = 0.0264

To get the answer the book has you'd have to use 60 and 50 which =/= 120.

Thank you for the help, I guess I should not practice from the book instead go on paspapers because first time I did this method but the answer didn't match and it confused me lol.

Thanks again :smile:

Reply 12

davros

16

Original post by ExWunderkind

I hate it when that happens, it does make you second guess yourself. But I would say it's a typo, unless I too am making a mistake.

0.50 \times 0.04 = 0.02

0.08 \times 0.08 = 6.4 \times 10^{-3}

0.02 + 6.4 \times 10^{-3} = 0.0264

To get the answer the book has you'd have to use 60 and 50 which =/= 120.

Unless they're doing (first order = 50 and second order = 70) + (first = 70 and second = 50) + (first = 60 and second = 60)

Reply 13

ExWunderkind

12

Original post by singh224

Thank you for the help, I guess I should not practice from the book instead go on paspapers because first time I did this method but the answer didn't match and it confused me lol.

Thanks again :smile:

Original post by davros

Unless they're doing (first order = 50 and second order = 70) + (first = 70 and second = 50) + (first = 60 and second = 60)

Yeah I think Davros has it. They are classing 50 + 70 and 70 + 50 as two separate combinations, which I guess makes sense.

And it does equal 0.0464.

S3 Discrete Random Variable.

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