Here's why:

* Beta decay involves the emission of a beta particle (an electron) from the nucleus of an atom. This electron is actually a neutron that decays into a proton and an electron.

* Potassium-42 has 19 protons and 23 neutrons.

* When potassium-42 undergoes beta decay, one neutron turns into a proton, increasing the atomic number by one.

* This results in calcium-42, which has 20 protons and 22 neutrons.

The reaction can be written as:

```

⁴²K → ⁴²Ca + e⁻ + νₑ

```

Where:

* ⁴²K is potassium-42

* ⁴²Ca is calcium-42

* e⁻ is the beta particle (electron)

* νₑ is an antineutrino