A northbound plane left Calgary at \( 7: 00 \mathrm{PM} \), flying at \( 800 \mathrm{~km} / \mathrm{hr} \). At the same time another plane left the same airport flying south at \( 750 \mathrm{~km} / \mathrm{hr} \). In how many hours will the two planes be 4650 kilometres apart?

To determine how many hours it will take for the two planes to be 4650 kilometers apart, we can analyze their movements as follows: 1. **Directions and Speeds:** - **Northbound Plane:** - Speed = 800 km/h - **Southbound Plane:** - Speed = 750 km/h 2. **Relative Speed:** Since the planes are moving in opposite directions (north and south), their speeds add up when calculating the rate at which the distance between them increases. \[ \text{Relative Speed} = 800 \, \text{km/h} + 750 \, \text{km/h} = 1550 \, \text{km/h} \] 3. **Time Calculation:** We need to find the time \( t \) it takes for the distance between them to reach 4650 kilometers. \[ \text{Distance} = \text{Relative Speed} \times t \] \[ 4650 \, \text{km} = 1550 \, \text{km/h} \times t \] \[ t = \frac{4650 \, \text{km}}{1550 \, \text{km/h}} = 3 \, \text{hours} \] **Answer:** It will take 3 hours for the two planes to be 4650 kilometres apart.