CDS 2026 · PYQ · Combinatorics / Statistics · medium
All possible groups of 3 distinct numbers from among A, B, C, D and E are formed. If the aggregate of sums of numbers of each group is 120, then what is the arithmetic mean of A, B, C, D and E?
A.4✓ Correct
B.5
C.10
D.20
Explanation
Number of groups of 3 from 5 = C(5,3) = 10. Each of A, B, C, D, E appears in C(4,2) = 6 groups. So aggregate = 6(A+B+C+D+E) = 120, giving A+B+C+D+E = 20. Arithmetic mean = 20/5 = 4.
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