The lengths of videos of a certain popular song have a normal distribution with mean 3.9 minutes. 18% of these videos last for longer than 4.2 minutes.
(i) Find the standard deviation of the lengths of these videos.
(ii) Find the probability that the length of a randomly chosen video differs from the mean by less than half a minute.
The lengths of videos of another popular song have a normal distribution with the same mean of
3.9 minutes but the standard deviation is twice the standard deviation in part (i)The probability that the length of a randomly chosen video of this song differs from the mean by less than half a minute is
denoted by p.
(iii) Without any further calculation, determine whether p is more than, equal to, or less than your answer to part (ii). You must explain your reasoning.
Sol. (i) I got s.d = 0.328
I am really confused and found diffcult on how to do on other two parts. Please help me out.