To find out how many trips Fred needs to cycle a total of 24 kilometers, we first need to determine how far he cycles per trip.
Fred made 3 trips to cycle a total of 18 kilometers. Thus, the distance he cycles per trip can be calculated by dividing the total distance by the number of trips.
\[ \text{Distance per trip} = \frac{\text{Total distance}}{\text{Number of trips}} = \frac{18 \text{ km}}{3 \text{ trips}} = 6 \text{ km per trip} \]
Now, we want to find out how many trips he needs to make to cycle 24 kilometers. We can use the distance per trip to find this:
\[ \text{Number of trips required} = \frac{\text{Total distance desired}}{\text{Distance per trip}} = \frac{24 \text{ km}}{6 \text{ km per trip}} = 4 \text{ trips} \]
Therefore, Fred will have to make 4 trips to cycle a total of 24 kilometers.