Ratio of girls to boys = 1 : 5, Number of girls = 239

Since the ratio of girls to boys is $1:5$, simply multiply the number of girls by $5$: $239 \times 5 = 1195$.

Students often divide the number of girls by $5$ instead of multiplying, or confuse the terms in the ratio resulting in an incorrect cross-multiplication.

In ratio problems involving sub-groups, always map the unit value first; here $1\text{ unit} = 239$, so $5\text{ units} = 5 \times 239$, a concept frequently used in mixture and partnership problems.