How to work out a Stratified Sample
just like working out a percentage you:
divide the number of the group you need a sample from by the total number of people and multiply the answer by the size of the sample you need to find.
There are 50 boys and 80 girls in a year group. You need to get a number of girls using a stratified sample of 30.
80/130 * 30 = 18.46153846
if you're unsure of which number to round it to, then find out the stratified sample of boys:
50/130 * 30 = 11.53846153
as this number is closer to 12, we would say the stratified sample of boys is 12 and the sample of girls is 18. 12 + 18 = 30 (just to prove you've got the right numbers)