You are Here: Home

# Stats question- Distributions watch

Announcements
1. Need a bit of explainign how this works
continuous random variable Y
mean=100
variance=256

Find K such that P(100-k more than /equal to Y less than eual to 100+k)
= 0.516

ans :k=11.2

Mark scheme says
p(Y less than/equal to 100 + k )= 0.516 + 1/2(1-0.516)
=0.758
P(Z less than/eual to k/16) = 0.758
and so on

i can't see how they arrived to these 2 conclusions

It also says some candidates stated
(100 +k -100) root 256
can be simplified to k/16 also

I'm not following this :s
from June 2001 paper
2. (Original post by MIZZ)
Need a bit of explainign how this works
continuous random variable Y
mean=100
variance=256

Find K such that P(100-k more than /equal to Y less than eual to 100+k)
= 0.516

ans :k=11.2

Mark scheme says
p(Y less than/equal to 100 + k )= 0.516 + 1/2(1-0.516)
=0.758
P(Z less than/eual to k/16) = 0.758
and so on

i can't see how they arrived to these 2 conclusions

It also says some candidates stated
(100 +k -100) root 256
can be simplified to k/16 also

I'm not following this :s
from June 2001 paper
The question is P(100-k<Y<100+k) = 0.516. As the distribution is normal, you know it must be symmetrical. As you know the area 0.516 is split evenly around the value 100 (which is the mean), there must be 0.258 on each side of 100. So on a standardised curve, phi of the z value would equal 0.5 + 0.258 = 0.758. If you look at the normal distribution table, z = 0.70 corresponds to this exactly. Thus by destandardising it (multiplying it by the standard deviation, which is 16) you should obtain k = 11.2.
3. ahhh
thanks ever so much boygenius

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 28, 2004
Today on TSR

### Edexcel GCSE Maths Unofficial Markscheme

Find out how you've done here

### OCR AS Chemistry Exam Discussion

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE