Final answer:
To calculate the change in speed required to arrive earlier at a destination, divide the distance by the new time and compare to the original speed. For 1, 2, 3 hours less, the speed adjustments are to 4 km/h, 6 km/h, and 12 km/h respectively. Four hours less is not possible as it implies arriving before starting.
Step-by-step explanation:
The question concerns how to adjust a walking speed to reduce travel time over a fixed distance. Specifically, a girl who walks 12 km at a speed of 3 km/h needs to increase her speed to arrive 1, 2, 3, and 4 hours earlier. The formula for speed is speed = distance / time. Currently, at 3 km/h, it takes her 12 km / 3 km/h = 4 hours. To take one hour less, she would need to walk the 12 km in 3 hours, which would mean a speed of 12 km / 3 h = 4 km/h.
For two hours less, she needs to do it in 2 hours, which equals 12 km / 2 h = 6 km/h. For three hours less, in 1 hour, so 12 km / 1 h = 12 km/h. Finally, for four hours less, this is not achievable as she can't arrive before she starts.
One can calculate the new speed by dividing the distance by the new time (original time minus the time less) and compare that to her original speed to find the necessary increase.