Marta walked 3.9 miles per hour for .72. how far did she walk

Sadly, you neglected to include the unit of the 0.72 .
Here are several possibilities:

0.72 seconds   ==>                   0.00078 mile
0.72 minutes    ==>                   0.0468 mile
0.72 hours       ==>                   2.808 miles
0.72 days        ==>                 67.392 miles 
0.72 weeks     ==>               471.74 miles
0.72 fortnights ==>              943.49 miles
0.72 years       ==>         24,614.9 miles
0.72 millenia    ==>  24,614,928 miles .

It have more energy picking up more particles in less time