What fraction of members were between 150 and 160 cm tall?

Height (cm) | Frequency (members)
150–160 & 18 \
160–170 & 39 \
170–180 & 58 \

Formula: total frequency = sum of all frequencies

Substitute:

Total number of members = 115 members.

Frequency = 18 members.

Formula: fraction = $\frac{\text{frequency in interval}}{\text{total frequency}}$

Check common factors:
18 = $2 \times 3^2$
115 = $5 \times 23$
GCD(18, 115) = 1

The fraction $\frac{18}{115}$ is already in its simplest form.

\dfrac{18}{115}