In which of triples the central number is strictly in the middle between two others.

Explanation

To determine which triple has the central number strictly in the middle between the other two, we can calculate the differences between each pair of adjacent numbers in the triples. Let's examine each option: ½ , ⅓ , ¼ Differences: ½ - ⅓ = ⅙ , ⅓ - ¼ = 1/12 The central number is not strictly in the middle. 12, 21, 32 Differences: 21-12=9 , 32 - 21=11 The central number (21) is not strictly in the middle. 3, 7, 13 Differences: 7−3=4, 13−7=6 The central number (7) is not strictly in the middle. ⅓, ½ , ⅔ Differences: ½ - ⅓ = ⅙ , ⅔ - ½ =⅙ The central number is strictly in the middle