Answer
a) $^8_5$B --> $^8_4$Be + $^0_{+1}$e
b) $^{15}_8$O --> $^{15}_7$N + $^0_{+1}$e
c) $^{40}_{19}$K --> $^{40}_{18}$Ar + $^0_{+1}$e
d) $^{13}_7$N --> $^{13}_6$C + $^0_{+1}$e
Work Step by Step
A positron emission occurs when a radioisotope breaks down into a new element/isotope plus a positron ($^0_{+1}$e). To balance this equation, the new element/isotope must have an atomic number of one less than the original isotope. The mass number stays the same.
