we know
distance =time *speed
from given question we have following equation
4*t+(1.5-t)*8=10
4*t+12-8t=10
12-4t=10
-4t=10-12
4*t=2
t=0.5 hour
hence at speed 4 miles per hour he crosses 4*0.5=2 miles
at speed 8 miles per hour he crosses 8*(1.5-0.5)=8 miles