Answer:
721.1 grams.
Explanation:
To determine the mass of the sample on the 8th day of the experiment, you can use the formula for compound interest, which is given by the following equation:
A = P(1 + r/n)^nt
where A is the final amount of the substance, P is the initial amount, r is the rate of decay, n is the number of time periods, and t is the number of time periods elapsed. In this case, the initial amount of the substance is 1400 grams, the rate of decay is 12%, and the number of time periods is 1 day. Therefore, the mass of the substance on the 8th day can be calculated as follows:
A = 1400(1 + 0.12/1)^(1*8) = 721.1 grams
Therefore, the mass of the radioactive sample at the beginning of the 8th day of the experiment is 721.1 grams.