Angle Between Two Planes

This is a VCE Specialist Mathematics question from 10minutequiz.com.
To find the angle between two planes, find the angle between the two vectors normal to the planes.
This video shows why the angle between planes is equal to the angle between the normal lines, and what to do in the case where the angle between the two normal vectors is obtuse (subtract from 180).
More VCE Mathematics resources at 10minutequiz.com

[@crvtbhununun6012]

Without visualising, how do we know if we want the obtuse angle or the acute angle? Thanks.

[@MathsWithMrLim]

Convention is to always give the acute angle