18) 5

15 take the BMAT
13 take the Biology
2 don’t take Biology
Hence the 2 that do not take Biology must be taking Chemistry, Physics and Maths.
Now, 3 people take Chemistry, Biology, Maths and Physics. This gives us 3+2=5.
We can assume all 5 of these people are boys because we are looking to maximise the number of boys.

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