The cost of a manicure is $25.
We want to find the 20% of $25. We have the following equivalence
20% → ?
100% → $25
If we divide each side of the equivalence we will have the same answer
[tex]\begin{gathered} \frac{20}{100}=\frac{?}{25} \\ \frac{1}{5}=\frac{?}{25} \end{gathered}[/tex]Multiplying both sides by 25 we have:
[tex]\begin{gathered} 25\cdot\frac{1}{5}=\text{?} \\ 5=\text{?} \end{gathered}[/tex]Then the person decides to leave $5 (that is the 20% of $25)
Then, the total cost including the tip is $25 + $5 = $30
