Given that d is the midpoint of ab and k is the midpoint of bc, which statement must be true?
db=bk
b is the midpoint of ac.
d bisects ak.
ak + bk = ac

All the statements are not true except:
 AK + BK = AC
Since BK = KC, so
AK + KC = AC

DB=BK is not necessarily true since it is not stated whether AB = BC
B is the midpoint of AC is not necessarily true since again it's not stated that AB = BC
D bisects AK is not necessarily true since it is not stated that D and K lie on the same line.

The answer is 225 miles per 4.5 hours. hope this ; )