Mistakes on Q4 were sign errors with Gaussian elimination mostly. I deducted marks for lambda = 1 ,-1 let to no solutions/infinitely many without justification, and a handfull tried using the expressions for x_1,x_2,x_3 to show it rather than looking at the row echelon form.

Mistakes on Q5 were a few people actually writing out the mutliplication as componentwise instead of summing over indices. I also removed a couple marks off if students wrote out the two matrices explicitly to show they multiply to the identity. As when the matrices are nice its pretty clear how they cancel and how someone could follow it, although of course it's not as rigorous as writing out the sums.

Mistakes on Q8 were virtually non existent. Although one person did it via induction on the size of the matrix, which was crazy but seemed sound!