To find the ratio of the number of female passengers to the number of male passengers, we can use the following steps:
- Identify the number of female passengers: There are 31 women on the airplane.
- Identify the number of male passengers: There are 13 men on the airplane.
- Set up the ratio of female passengers to male passengers:
\[ \text{Ratio} = \frac{\text{Number of Female Passengers}}{\text{Number of Male Passengers}} = \frac{31}{13} \]
Thus, the ratio of the number of female passengers to the number of male passengers is 31:13.