Which of the following is true about I.Q?

Concept: Intelligence Quotient (IQ), introduced by William Stern and used in tests such as the Stanford-Binet scale, is computed as IQ = (Mental Age ÷ Chronological Age) × 100 — a ratio comparing a person's mental ability to the norm for their actual age, expressed as a percentage.

Application: In this ratio, Mental Age sits in the numerator and Chronological Age sits in the denominator. Holding Chronological Age fixed, a rise in Mental Age enlarges the ratio, so IQ moves in the same direction as Mental Age — a direct relationship. Holding Mental Age fixed, a rise in Chronological Age enlarges only the denominator, which shrinks the ratio, so IQ moves in the opposite direction to Chronological Age — an inverse relationship. This matches the claim that IQ is inversely related to Chronological Age.

Cross-check: Take a child with Mental Age 10 and Chronological Age 10 — IQ = (10/10) × 100 = 100. If Mental Age stays at 10 while Chronological Age grows to 12 with no matching cognitive growth, IQ = (10/12) × 100 ≈ 83, confirming IQ fell as Chronological Age rose. If instead Mental Age grows to 12 while Chronological Age stays at 10, IQ = (12/10) × 100 = 120, confirming IQ rose alongside Mental Age. This rules out a same-direction relationship with Chronological Age and rules out a "directly related to both" claim.