Section Average Marks and Student Difference

CAT 2022 Slot 3 · QA · Hard · Arithmetic

In an examination, the average marks of students in sections A and B are 32 and 60, respectively. The number of students in section A is 10 less than that in section B. If the average marks of all the students across both the sections combined is an integer, then the difference between the maximum and minimum possible number of students in section A is

Answer

63

Explanation

Let nn be the number of students in section A. Then section B has n+10n + 10 students. Total students =2n+10= 2n + 10. Combined average AavgA_{avg}: Aavg=32n+60(n+10)2n+10=92n+6002n+10=46+1402n+10A_{avg} = \frac{32n + 60(n + 10)}{2n + 10} = \frac{92n + 600}{2n + 10} = 46 + \frac{140}{2n + 10}

For AavgA_{avg} to be an integer, 1402n+10\frac{140}{2n + 10} must be an integer, which means 2n+102n + 10 must be a divisor of 140. Since n1n \ge 1, 2n+10122n + 10 \ge 12, and 2n+102n + 10 is even.

Divisors of 140 that are even and 12\ge 12: 14,20,28,70,14014, 20, 28, 70, 140.

Corresponding values of 2n+102n + 10:

  • 2n+10=14    n=22n + 10 = 14 \implies n = 2
  • 2n+10=20    n=52n + 10 = 20 \implies n = 5
  • 2n+10=28    n=92n + 10 = 28 \implies n = 9
  • 2n+10=70    n=302n + 10 = 70 \implies n = 30
  • 2n+10=140    n=652n + 10 = 140 \implies n = 65

Maximum possible n=65n = 65. Minimum possible n=2n = 2.

Difference =652=63= 65 - 2 = 63.

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