- #1

mutineer123

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## Homework Statement

http://www.xtremepapers.com/CIE/International%20A%20And%20AS%20Level/9709%20-%20Mathematics/9709_s05_qp_6.pdf

question no. 6 part ii (Safety regulations state that the pressures must be between 1.9 − b bars and 1.9 + b bars. It is

known that 80% of tyres are within these safety limits. Find the safety limits)

My answer is not matching anywhere near the right answer (http://www.xtremepapers.com/CIE/International%20A%20And%20AS%20Level/9709%20-%20Mathematics/9709_s05_ms_6.pdf)

## Homework Equations

## The Attempt at a Solution

I used .8 as my probability for which the Z value is .842

Then I standardised my X values to Z, so i got P( -b/0.15< Z < b/.15) =0.8

Computing that i got .06315 as my b value.

Where is the answer sheet getting ± 1.282 from?