Answer:
15 miles per hour
Step-by-step explanation:
12 miles per hour / 60 mins = 0.2 miles per minute
0.2 mpm * 10 mins = 2 miles (it takes her 10 mins to travel 2 miles)
at to 2 minutes shes already 0.4 miles ahead of her brother.
he will have to travel to school in 8 mins to arrive at the same time as his sister so his speed will have to be x*8= 2 so x=2/8 or 0.25 so his speed will have to 0.25 mpm so 0.25*60= 15
his speed should be 15 miles per hour
