The average age of 3 children in a family is 30% of the average age of mother and the eldest child. The total age of father and the youngest child is 45 years. If the mother‘s age is 40 and average age of father and mother is 42, what is the age of second child?

Given: 3 children's average = 30% of (mother + eldest child's average). Father + youngest child = 45 years. Mother = 40, father + mother average = 42. From father + mother = 84, we get father = 44. Then youngest child = 45 - 44 = 1. The condition becomes: (C1 + C2 + C3)/3 = 0.3 × (40 + C3)/2, where C3 is eldest. This simplifies to C1 + C2 + C3 = 0.45(40 + C3), or C1 + C2 = 18 - 0.55C3. For valid positive ages, no unique solution exists without more information.