Don't quite understand

You need to find the first common multiple of 3, 4 and 6. This is 12. This will be the first common meeting at the library after the meeting on the 1st of April.

So, they will all meet again on the same day after 12 days from the 1st of April. This would be the 13th of April.

You can work out for the earlier days that not every one of them will be present on those earlier days between the 1st of April and the 13th of April.

The tutor has done the question well in my opinion.

Ok understand now. Thanks