Answer
See explanation.
Work Step by Step
a. The particle emitted in $\beta^{-}$ decay is an electron. By conservation of charge, the daughter nucleus has a Z that is higher by 1, so it is yttrium, $^{90}_{39}Y$, with Z = 39.
b. In a time of one half-life, the number of radioactive nuclei decreases by a factor of 2.
56 years is 2 half-lives, so $\frac{1}{4}$ is left.
c. 6.25$\%$ is $\frac{1}{16}$, so 4 half-lives must have elapsed. It will take (4)(28y)=112 years.
