$C_1 V_1 = C_2 V_2$ has four sockets, and the exam rotates through them all: how much stock to pour $\left(V_1 = \dfrac{C_2 V_2}{C_1}\right)$, what a mystery stock must have been $\left(C_1 = \dfrac{C_2 V_2}{V_1}\right)$, what a dilution lands at. Keep the subscripts honest — 1 is the strong start, 2 is the watery finish — and every rearrangement is one division away.

The twist this lesson exists for: $V_2$ is the final total volume, not the water you pour. The flask already holds $V_1$ of stock before the wash bottle comes out, so the water added is $V_2 - V_1$ — the difference, never the total. Real procedures say “dilute TO 500 mL”, not “add 500 mL”, and the word to is doing heavy lifting. Read for it.