Based on the fraction 5/6, the number of pupils in XYZ at first is "6 units".

Based on the fraction 1/4, the number of pupils in XYZ at first is "4 units".

These do not match. You must make the same number of units, say 12 units or 24 units.

So, I convert 5/6 to 10/12 and 1/4 to 3/12.

Meaning to say, at first,
- ABC has 10 units of students
- XYZ has 12 units of students

Then, 3 units of students moved from XYZ to ABC, and after that,
- ABC has 13 units of students
- XYZ has 9 units of students, corresponding to a figure of 450 students

9 units --> 450 students
1 unit --> 50 students
10 units --> 500 students

So, there are 500 students in ABC at first.

Alternatively, you can do a backward approach to this question.

3/4 of the students in XYZ, or 450 students, remained in the school.

You can easily calculate this to 600 students in XYZ at first.

You know how to find ABC from there.

thank u