Comment: This problem can be paraphrased into one involving selecting 4 (or 6) objects from a given 10 objects (from this perspective, this problem is one that involves combination, which is required in the syllabus). It can also be seen as a problem involving the arrangement of 10 objects, 4 of which are alike of the first kind and 6 of the second kind. This perspective is not required in the syllabus.