Maths probability??

year10 group year 11 group


boys 75 50


girls 60 30





above, shows the number of boys and the number of girls in year10 and year11 of a school.


The headteacher wants to find out what pupils think about a new year 11 common room. A stratified sample of size 50 is to be taken from year 10 and year 11.





a) calculate the number of pupils to be sampled from year 10.





two pupils are to be chosen at randon to speak to the headteacher.





one pupil is to be chosen from year 10.


one pupil is to be chosen from year 11.





b) calculate the probability that both pupils will be boys.





however, the headteacher decides to choose one boy at random from all the boys in year 10 and 11 together and one girl at random from all the girls in year 10 and 11 together.





c) calculate the probability that exactly one of the pupils will be from year 10.

Maths probability??
A) Approximately 60% of the students are in group 10 so 60% of the sample should come from year 10. 60% of 50 is 30.





B) (75/135)*(50/80) is approximately 34%





C) (75/125)(30/90) + (50/125)(60/90) is approximately 47%