Family ages with youngest at birth: The average age of five sisters is 20 years at present. The youngest sister is currently 4 years old. What was the average age of the full group (all five, counting the newborn as age 0) at the exact time of the youngest sister’s birth?

Introduction / Context:
Average age problems often require translating “average now” into a total sum and then rolling ages backward by a fixed number of years. Here, we compute the group average at the moment the youngest was born by deducting elapsed years from the older siblings and setting the newborn's age to 0.

Given Data / Assumptions:

• Five sisters in total.
• Present average age = 20 years.
• Youngest sister's present age = 4 years.
• At the time of birth of the youngest (4 years ago), the other four were each 4 years younger; the youngest was 0.

Concept / Approach:
Convert the present average to a present total. Move 4 years back in time: subtract 4 years for each of the four elder sisters; the youngest contributes 0 at birth. Finally, divide by 5 to get the past average.

Step-by-Step Solution:

Verification / Alternative check:
Forward check: Add 4 years to each of the five to get from birth to now adds 20 years to total, consistent with the present totals computed above.

Why Other Options Are Wrong:

• 15 years: Would imply a total of 75 at birth, inconsistent with present data.
• 17 or 18 years: Overestimates the elders’ contribution after rolling back ages.

Common Pitfalls:
Forgetting to reduce each elder’s age individually by 4 years or treating the newborn as already aged 4 at birth.

Final Answer: