Nina can ride her bike 63,360 feet in 3,400 seconds, and sophia can ride her bike 10 miles in 1 hour. what is nina's rate in miles per hour if there are 5,280 feet in a mile? which of these girls bikes faster?

We know that 
3600 seconds = 1 hour
Then
3400 seconds = 3400/3600
                        = 17/18 hours
5280 feet = 1 mile
Then
63360 feet = 63360/5280 mile
                  = 12 mile
So
Distance covered by Nina in 17/18 hours = 12 mile
Then
Distance covered by Nina in 1 hour = 12/(17/18) miles
                                                         = 12.71 miles.
From the above deduction, we can conclude that the speed of Nina is 12.71 miles per hour.
Speed of Sophia is 10 miles per hour. So Nina is the girl that rides at a faster speed.