found another mistake in a different question,
before that small note: the mistake above is about Q4 not Q5.
mistake in Q3:
max(z,b) on sum(wi*(z^t*xi-b-yi)^2
max should be min (easy to see, most simply than b=infinity is the solution),
also I don't see why z^t instead of z (or a in the presentation) because this is a scalar so the transpose is meaningless.
since there was a mistake in both linear programming answers I'm going to post a request to the course staff, in the "Isn't the test too long ?" open thread in a couple of minutes.