Let the age of the grandchild be x ⇒ Age of the mother = 2x ⇒ Age of the grandfather = 3 × (2x) = 6x ⇒ Average = Sum of three ages/3 ⇒ 51 = (x + 2x + 6x)/3 ⇒ 51 = 9x/3 ⇒ x = 17 ⇒ Age of grandfather = 6x = 102 years ⇒ Age of grandchild = x = 17 years ∴ Required difference = 102 – 17 = 85