Hi there! 9.125 * 10^-3 is definitely less than 9.125 * 10^2, because when it comes to negative powers of 10, the numbers are decimals. 10^-3 equals 0.001 or one thousandths in word form. 10^2 is 100. When solving for the answers. 0.009125 is less than 912.5. When 912.5 is divided by 0.009125 912.5 is 100^5 or 100,000 times greater than 0.009125. The answer is that 9.125 * 10^-3 is 100,000 (100^5) times less than 9.125 * 10^2.
The different isotopes have different "half-lives" – the time it takes to lose half of its radioactivity. Pu-239 has a half-life of 24,100 years and Pu-241's half-life is 14.4 years. Substances with shorter half-lives decay more quickly than those with longer half-lives, so they emit more energetic radioactivity.