Answer
See work below.
Work Step by Step
An einsteinium nucleus has 99 protons and a fermium nucleus has 100 protons.
a. Fermium can decay into einsteinium if it captures an electron or emits a $\beta^{+}$. In effect, a proton is converted into a neutron.
b. Einsteinium can decay into fermium if it emits a $\beta^{-}$. In effect, a neutron is converted into a proton.
