Concentration is bookkeeping: $C = \dfrac{n}{V}$ — read aloud C equals n over V — where $C$ is the molar concentration in mol/L, $n$ is the amount of solute in moles, and $V$ is the volume of the finished solution in litres. Not the volume of solvent you poured: the volume of what is in the flask when you stop.

Now add water. The solute does not care — no moles arrive, none leave — so $n$ is conserved while $V$ grows, and $CV$ before must equal $CV$ after: $C_1 V_1 = C_2 V_2$, read aloud C-one V-one equals C-two V-two. State the subscript convention once and never wobble: 1 is BEFORE the water — the stock you pipetted — and 2 is AFTER — the finished solution. Because the two volumes meet as a ratio, millilitres cancel millilitres and nothing needs converting.

The trap this lesson exists to prevent: $V_2$ is the final total volume, not the water added. Dilute 50 mL up to 300 mL and you added 250 mL of water, not 300. Read the ask twice — one word decides which number gets the mark.