Found a really weird problem. Mechanics I, January series, OCR board question 7, (ii) part (b)

A particle P of mass 0.6 kg is projected up a line of greatest slope of a plane inclined at 30 degrees to the horizontal. P moves with deceleration 10 metres per second squared and comes to rest before reaching the top of the plane.

Find the magnitude of the contact force exerted on P by the plane and the angle between the contact force and the upward direction of the line of greatest slope when P is at rest.

The mark scheme for this exam paper reckons that the contact force exerted on P by the plane will be vertical when P is at rest. How do they arrive at this conclusion?