You have spent a chapter adding resistors in series and taking product-over-sum in parallel. Capacitors do both of those things, in the opposite places. That is the whole lesson, and the only way to keep it is to understand WHY rather than to memorise WHICH.

Fix the subscripts once: $C_1$ and $C_2$ are the two capacitors as marked on their bodies, in farads, and $C_t$ is the single capacitance the rest of the circuit actually sees. In parallel, $C_t = C_1 + C_2$ — C-total equals C-one plus C-two. Two capacitors side by side are one capacitor with both lots of plate area, and area only ever adds. In series, $C_t = \dfrac{C_1 C_2}{C_1 + C_2}$ — C-total equals C-one C-two over C-one plus C-two. Stacking them end to end effectively widens the gap between the outer plates, and a wider gap stores less.

So the sanity rails point the other way from the resistor ones. Parallel lands above the larger of the two. Series lands below the smaller — below BOTH, not between them. Check every answer against those two sentences before you check the arithmetic; a swapped rule fails the rail instantly, and it is the mistake this lesson exists to catch.

One nugget for the field: series capacitors are how a bank is built to survive a voltage no single capacitor is rated for, and the price of that trick is exactly the capacitance the series rule takes away.