Question 98923
This is the variation of problem I was already solving. So look at problem 73154 for some info. Anyway, crucial idea - times of flight of both planes are same -) t1=t2 s = v/t v*t=s

v1 = x
v2 = x+80

this is what we substitute into our equations

v1*t=s1
v2*t=s2
s1+s2 = 2100

and get:

x*t=s1
(x+80)t = s2
s1+s2 = 2100

x*t+(x+80)*t = 2100
x*t+x*t+80t = 2100
2xt=2100-80t
2x= (2100-80t)/t
x = (2100-80t)/2t
- (analytic solution)
-
x= (2100-80*2.5)/5
x= (2100-200)/5
x= 1900/5
x= 380 miles/hour