1. The problem statement, all variables and given/known data Metal strips are produced mean length 150cm, standard deviation 10cm. Find probability that length of random strip is: a/ shorter than 165cm b/ longer than 170cm c/ between 145 and 155cm 2. Relevant equations Z = x - μ / σ θ (Z) - θ (z) = average probability (question c) 3. The attempt at a solution a/ Z = 165 - 150 / 10 = 1.5 1.5 from standardised table = 0.9332 = 93.32% probability length shorter than 165cm b/ Z = 170 - 150 / 10 = 2 2 from standardised table = 0.9772 Now here I did 1 - 0.9772 = 0.0228 = 2.28% probability length longer than 170cm. I did this not because I understand the curve graph, but because without subtracting 1 the probability was obviously wrong. c/ as per a/ to get Z = -0.5 for 145cm and Z = 0.5 for 155cm used tables to get 0.6915 for 0.5. Then did: θ = 0.6915 - (1 - 0.6915) = 0.383 = 38.3% between 145 and 155 cm Again, I had to -1 to make equation work anyway, BUT also to give a sensible probability. I'm assuming answers are right? But how do we say when to -1 or not. I know it' to left or right of mean etc, but simply can't get head round it? Any help welcome as always! Thanks guys. Oh I know θ should be O with vertical line, but couldn't find it on the selection box, so made do..