How do I do this one?I made a probability tree diagram and added the probability of the pupils being right handed but after that I'm not quite sure what to do,any help?
\(N_{left} = N_{girl}\cdot P[\text{girls are left handed}]+N_{boy}\cdot P[\text{boys are left handed}]\\ N_{left} = 320 \cdot 0.2 + 250 \cdot 0.1\\ N_{left} = 64+25 = 89\)
+134 
