<PRE><B>Question 175420</B>
Let x = speed of the slower plane
x+100 = speed of the faster plane


Distance = Rate * Time

4(x+100)=6x
4x+400=6x 
400=2x
x= 200 mph slower plane
x+100 = 300 mph faster plane


Check:  Faster plane (300 mph) in 4 hours travels 1200 miles
Slower plane (200 mph) in 6 hours travels 1200 miles.


R^2</PRE>