In the edexcel spec of S1 you have to work out mean and std. dev. from the Q1 and Q3quartiles, I know how to solve via two simultaneous equations etc. but when given quartiles the values for probability distribution (on the second smaller table) do not contain 0.75 or 0.25 as the probability for Φz (how do I get around this? Average the number between 0.3 and 0.2 for 0.25 or take the close to 0.75 I can find on the first table i.e. 0.7486? This is all to get the correct z value.

Thanks for any help