Average rate of loss from days 10 to 13 = (Level day 10 - Level day 13) / 3 days.<br><br>
Days from day 13 to day 28 = 15 days.<br>
Projected additional loss = Rate × 15.<br>
Projected level at day 28 = Level at day 13 - Projected additional loss.<br><br>
Compare: Actual reading on day 28 (from graph) vs projected level.<br><br>
Final Answer: Read from the graph and compute.