It'll be easier doing this as improper fractions.
15 3/4= 63/4
To find a fraction of another number, we have to multiply them
(2/3)(63/4)
126/12
Then, we want to make this a mixed number again so it's easier to tell what 126/12 actually means.
126/12 = 10 6/12 = 10 1/2
In conclusion, Brad has gone 10 1/2 miles