A Biologist Measures the Amount of Contaminant in a Lake $Q$

Short Answer

A biologist measures the amount of contaminant in a lake 2 hours after a chemical spill and again 15 hours after the spill. She sets up a possible model to determine $Q$ , the amount of the chemical remaining in the lake as a function of $t$ , the time in hours since the spill. The model assumes the contaminant is leaving the lake at a constant rate, which she determines to be 6 tons\hour. She estimates that the lake will be free from the contaminant 35 hours after the spill. How many tons of the contaminant were in the lake at the 15 hour reading?