Two brothers marco and fernando are training for a marathon marco runs at a rate of 6miles per hor. fernando funs at at a rate of 7.2 miles per hour in the same direction. how much time will they be 0.3 miles apart

We know that
speed=distance/time
solve for time
time=distance/speed

in this problem
Marco runs at a rate of 6 miles per hour. 
Fernando funs at a rate of 7.2 miles per hour
Difference=7.2-6=1.2 miles/hour
so
speed=1.2 miles/hour
distance=0.3 miles
time=?
time=distance/speed> 0.3/1.2> 0.25 hour> 0.25*60=15 minutes

the answer is
0.25 hour (15 minutes)

Alternative Method
Let 
x> Fernando's distance when Marco is 0.3 miles apart

Fernando funs at a rate of 7.2 miles per hour
for distance =x
time=x/7.2> equation 1

Marco runs at a rate of 6 miles per hour. 
for distance=x-0.30
time=(x-0.30)/6> equation 2

equate equation 1 and equation 2
 7.2*(x-0.3)=6x> 7.2x-2.16=6x
7.2x-6x=2.16> x=2.16/1.2> x=1.8 miles

time=x/7.21.8/7.2=0.25 hour