The number of neutrons in an atom of ⁴⁰₂₀Ca is:

**The reasoning** Let's decode the notation ⁴⁰₂₀Ca. The **top number (40)** is the **mass number (A)** — total protons + neutrons. The **bottom number (20)** is the **atomic number (Z)** — number of protons (which defines the element). The principle: **Number of neutrons = Mass number − Atomic number** So: Neutrons = 40 − 20 = **20 neutrons** Think of it like this: the atom's total "mass participants" (40) minus the protons (20) leaves you with neutrons (20). **Why the wrong options tempt you** - **B (40)**: You might pick the mass number directly, forgetting it's the *sum* of protons and neutrons, not neutrons alone. - **C (60)**: This trap catches students who *add* 40 + 20 instead of subtracting. - **D (10)**: Perhaps you divided 20 by 2 randomly, but there's no such rule here. **Quick takeaway** Neutrons are the "leftovers" after you remove protons from the mass number: **N = A − Z**. Always subtract the bottom from the top!