A shop sells only 10 µF capacitors. You need a combination of 25 µF and want to use as few…

Step 1. Three capacitors are not enough. Three can make $30$, $15$, $6.7$ or $3.3\ \mu\text{F}$ and so on. None of these is $25\ \mu\text{F}$.

Step 2. Four capacitors. Write $25\ \mu\text{F} = 20\ \mu\text{F} + 5\ \mu\text{F}$. Two capacitors in parallel give $10\ \mu\text{F} + 10\ \mu\text{F} = 20\ \mu\text{F}$. Two in series give $\dfrac{10\ \mu\text{F} \times 10\ \mu\text{F}}{10\ \mu\text{F} + 10\ \mu\text{F}} = 5.0\ \mu\text{F}$.

Step 3. Join the two parts in parallel. $20\ \mu\text{F} + 5.0\ \mu\text{F} = 25\ \mu\text{F}$.