divdude, your algo is ok but it will take at least 3t hours, and, as i said earlier, t=10hrs in the question. Also, the total time available is T=10 hrs only. Rats are to be made drink water samples at the beginning of the 10 hr period and observations are to be made at the end of the period. No intermediate observations or conslusions are envisioned. [Hint: Think Digital]