Based on the Edexcel syllabus.
I have resolved the error. The question says 'standard deviation of at least 4' and the inequality should therefore read np(1-p)<=16 --> 20(1-p)<=16 which leads to p<=0.2.
Thanks for these excellent notes - I've been working through them in preparation for teaching S2 next year. A couple of minor things which i think are incorrect (but which you might correct me on!):
- slide 11 part c should be 8C4 not 8C3?
- slide 32 Q3 - I can't figure out what's wrong with this question! The first inequality should lead to p<=0.8 but in the context of the question that doesn't make sense, and would therefore lead to p=0.5 (2 options per question) and n=40 which gives s.d. of 10. Anyway, maybe I'm missing something more obvious but I just can't seem to resolve this.