# S2 Jan 2015 IAL, Question 1 B

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#1
http://pmt.physicsandmathstutor.com/...%20Edexcel.pdf
I don't understand why they write it like P(Y >= 1) WHAT IS Y? its making me want to tear my hair out
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3 years ago
#2
The ideal answer to 1B would start "Let Y = the number of cars caught in a 48 hour period. The distribution of Y is Y~Po(1.6)". They just happened to have chosen Y. You could use any letter as long as you define your random variable and state its distribution.
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#3
(Original post by old_engineer)
The ideal answer to 1B would start "Let Y = the number of cars caught in a 48 hour period. The distribution of Y is Y~Po(1.6)". They just happened to have chosen Y. You could use any letter as long as you define your random variable and state its distribution.
ok but why is it >= 1 because it specifically says that a car has been caught speeding right? So doesn't that mean that first we gotta take into account the fact that a car has been caught speeding already and then they ask to find probability that the period of time that passes before the next car is caught is less than 48 hours, why is it like this? shouldnt there be something like P(Y<2) where (48 hours = 2 days) where the random variable Y is defined as something, im so confused
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3 years ago
#4
I think it's possibly the way the question is asked that's causing the confusion here. When the question says "Find the probability that the period of time that elapses before the next car is caught speeding by this camera is less than 48 hours", the examiner is expecting you to interpret that as "Find the probability that at least one car is caught speeding within the next 48 hours". If that's still not clear, consider the implications if no cars were caught within the next 48 hours. In that case, the elapsed time until the next car was caught would be more than 48 hours, which is the opposite of what we've been asked to find the probability of.
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#5
OHH i understand now, they phrase it so badly it makes my brain hurt. Thank you very much for taking the time to explain this
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3 years ago
#6
I don't think the question is badly worded, necessarily. It's probably more to the point that the examiner is challenging you interpret a "real world" situation in mathematical terms rather than just telling you to calculate P(Y>=1) or whatever. I'm glad you understand it now, anyway, and hopefully now the marking guide makes sense.
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