Maths : Basic Numeracy

Q 47 / 370

UPSC CSE Prelims 2025

Let p + q = 10, where p, q are integers.
Value-I = Maximum value of p × q when p, q are positive integers.
Value-II = Maximum value of p × q when p ≥ 6, q ≥ 4.
Which one of the following is correct?

EXPLANATION

Correct Option (2)

The problem requires comparing two maximum values, Value-I and Value-II, derived from the equation p + q = 10, where p and q are integers.

Calculation of Value-I:

Value-I represents the maximum value of p × q when p and q are positive integers such that p + q = 10.

The pairs of positive integers (p, q) that sum to 10 are:

  • (1, 9) → p × q = 1 × 9 = 9
  • (2, 8) → p × q = 2 × 8 = 16
  • (3, 7) → p × q = 3 × 7 = 21
  • (4, 6) → p × q = 4 × 6 = 24
  • (5, 5) → p × q = 5 × 5 = 25
  • (6, 4) → p × q = 6 × 4 = 24
  • (7, 3) → p × q = 7 × 3 = 21
  • (8, 2) → p × q = 8 × 2 = 16
  • (9, 1) → p × q = 9 × 1 = 9

The maximum product obtained is 25. Therefore, Value-I = 25.

Calculation of Value-II:

Value-II represents the maximum value of p × q when p and q are integers such that p + q = 10, with additional constraints p ≥ 6 and q ≥ 4.

We examine integer pairs (p, q) that satisfy all given conditions:

  • If p = 6: q = 10 - 6 = 4. This pair (6, 4) satisfies p ≥ 6 and q ≥ 4. The product p × q = 6 × 4 = 24.
  • If p = 7: q = 10 - 7 = 3. This pair (7, 3) satisfies p ≥ 6 but does not satisfy q ≥ 4. Thus, it is an invalid pair.
  • For any p > 7 (e.g., p = 8, 9, 10), the corresponding value of q (10 - p) will be less than 3, failing the condition q ≥ 4. Hence, no further valid pairs exist.

The only valid pair satisfying all conditions is (6, 4), yielding a product of 24. Therefore, Value-II = 24.

Comparison:

Comparing the calculated values:

  • Value-I = 25
  • Value-II = 24

It is evident that Value-II < Value-I (24 < 25).

Incorrect Options:

Option 1 (Value-I < Value-II): This is incorrect because 25 is not less than 24.

Option 3 (Value-I = Value-II): This is incorrect because 25 is not equal to 24.

Option 4 (Cannot be determined due to insufficient data): This is incorrect as both Value-I and Value-II can be precisely determined from the given information and constraints.

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