Terrance and jessie are training for long-distance race terrance trains at a rate of 6 miles every half hour and jesse trains at a rate of 2 miles every 15 minutes what is the unit rate in miles per hour for each runner

There are several information's already given in the question. Based on those information's the answer can be easily deduced. 

Distance covered by Terrance in 1/2 hour = 6 miles
So
Distance covered by Terrance in 1 hour = 6 * 2 miles
                                                               = 12 miles
So the unit rate of Terrance is 12 miles per hour.

Now
Distance covered by Jessie in 1/4 hour or 15 minutes = 2 miles
Then
Distance covered by Jessie in 1 hour = 2 * 4 miles
                                                           = 8 miles
Then the unit rate of Jessie is 8 miles per hour.