A biologist has a 987-gram sample of a radioactive substance. Find the mass of the sample after two hours if it decreases according to a continuous exponential decay model, at a relative rate of $12\mathrm{\%}$ per hour.
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Do not round any intermediate computations, and round your answer to the nearest tenth.

Step 1 ：The problem is asking for the mass of the sample after two hours given that it decreases according to a continuous exponential decay model, at a relative rate of 12% per hour. The formula for continuous exponential decay is given by: $$P(t)=P_0\ast e^{(-rt)}$$ where: $P(t)$ is the final amount, $P_0$ is the initial amount, $r$ is the rate of decay, and $t$ is the time.

Step 2 ：In this case, $P_0=987$ grams, $r=0.12$ per hour, and $t=2$ hours. We can substitute these values into the formula to find the final amount.

Step 3 ：Substituting the given values into the formula, we get: $$P(t)=987\ast e^{(-0.12\ast 2)}$$

Step 4 ：Solving the above expression, we find that $P(t)=776.4$

Step 5 ：Final Answer: The mass of the sample after two hours is $\boxed{{\displaystyle 776.4}}$ grams.