...distrubituion with mean of 50cm. if a lace is...? ...greater than 53cm, it is classified as oversized and sold as a 55cm lace

smaller than 47cm, it is classified as undersized and sold as a 45cm lace

between 47cm and 53cm, it is classed as adequate and sold as a 50cm lace

If 90% of the laces are adequate, find:
a) the standard deviation
b) the percentage of oversize laces
c) probability of a lace being adequate, given that it is not undersize



OK. so, i've tried it, i can't get the answer. And please don't think that i'm asking you to do my homework - this is just one of many revision questions for which i have the answers to - it just doesn't show me how to do it.

please show working, i actually don't know where to start, but i know that the normal must be converted to the standard normal using


z = (x-mean)/standard deviation

but thats it.

thanks in advance!