Human body composition: Approximately what percentage of an average adult human body is water by weight?

Introduction:
Water is the dominant component of the human body and underpins circulation, cellular function, and temperature regulation. Estimating body water percentage is a basic biological fact relevant to health sciences and environmental engineering where human exposure and hydration are discussed.

Given Data / Assumptions:

• Average healthy adult, mixed body composition.
• Estimates rounded to broad percentages.
• No special conditions such as extreme obesity or dehydration.

Concept / Approach:
Total body water spans intracellular and extracellular compartments. Typical averages cite about two thirds of body mass as water; many references summarize this as roughly 60 to 70 percent depending on sex, age, and body fat. For a broad recall question, 70 percent is a commonly taught value highlighting the predominance of water in tissues.

Step-by-Step Solution:
Recall the conventional range for adults: about 60 to 70 percent.Select the representative midpoint used in many textbooks and quizzes.Therefore, choose 70 percent as the standard answer for average adult context.

Verification / Alternative check:
Physiology texts show total body water approximately 0.6 to 0.7 times body weight. Variations by sex and age shift the exact number, but 70 percent remains a widely accepted rounded figure.

Why Other Options Are Wrong:
20 percent and 40 percent are far below physiological norms. 85 percent is characteristic of certain organisms or infants, not average adults. 55 percent understates typical adult values used in general education.

Common Pitfalls:
Applying a single exact number to all populations. The exact percentage is context dependent; the question seeks a standard rounded estimate.

Final Answer:
70 percent