1. The problem statement, all variables and given/known data http://www.xtremepapers.com/CIE/International A And AS Level/9709 - Mathematics/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 A And AS Level/9709 - Mathematics/9709_s05_ms_6.pdf) 2. Relevant equations 3. 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?