Weekly temperature average with a missing day: The average temperature of the first three days of a week is 25° C and that of the next three days is 27° C. If the average for the whole week is 26.5° C, what is the temperature on the seventh day?

Introduction / Context:
Find the single missing value from grouped averages within a 7-day week.

Given Data / Assumptions:

• Average days 1–3 = 25° C
• Average days 4–6 = 27° C
• Average days 1–7 = 26.5° C

Concept / Approach:
Compute totals from averages for both 3-day blocks and for all 7 days, then subtract to get day 7.

Step-by-Step Solution:
Sum days 1–3 = 3 * 25 = 75Sum days 4–6 = 3 * 27 = 81Sum days 1–7 = 7 * 26.5 = 185.5Day 7 = 185.5 - (75 + 81) = 29.5° C

Verification / Alternative check:
Check that the combined totals match the weekly average after inserting 29.5 for day 7; they do.

Why Other Options Are Wrong:
31.5° C is too high; 21° C too low; 27.5° C and 26.0° C do not satisfy the weekly average constraint.

Common Pitfalls:
Using 6 rather than 7 days for the weekly total; not converting averages back to totals first.

Final Answer:
29.5° C