Explanation:

P scored 40 marks more than Q. So, marks of Q are q, then the marks of P will be q + 40.
Q scored 10% less marks than P. That is, marks of Q are 90% of the marks of P.
So, q = 90% of (q + 40)
Or 10q = 9q + 360
Or q = 360
So, Q scored 360 marks.