D
1) Keeping in mind that the equation is 7x +1 = 10x
7x ---> Refers to 7 times x,
1 ---> Constant
10x
2) Noticing that x can be "miles per hour" So we can write
Brody: 7x + 1 1st day
Brody: 10x 2nd day
And 7, and 10 are the number of miles, and x the number of hours:
7x +1 = 10x
7x -10x = -1
-3x =-1
x=1/3 hour
3) Hence, the answer is D