Answer
$4.5\;mL$.
Work Step by Step
The given values are
Base $B=150\;mL$.
Rate $R=3$.
Let the pure alcohol is $P=x$
The formula is
$\Rightarrow \frac{P}{B}=\frac{R}{100}$
Substitute all the values.
$\Rightarrow \frac{x}{150\;mL}=\frac{3}{100}$
Cross multiply.
$\Rightarrow 100x=450\;mL$
Divide both sides by $100$.
$\Rightarrow x=4.5\;mL$.
