Typically these motion problems are used with a formula to calculate rate and time,
d=rt
You want to assign each plane its own little table to make things easier
plane 1 lets call it plane F for faster
and plane 2 let us call it S for slower
remember to define x as the time they will both be 1400 miles apart.
F - we are looking for the time so that is our variable x
F - we know the rate is 600mph
S - we are looking for the time so that is our variable x
S - lets assume this is a bigger plane that flies slower at 450mph
d = rt
F =600*x
S =450*x
so now that we have rate and distance we can calculate the time.
600x + 450x = 1400 (the amount they will be apart) and the equation to solve for the x (time)
1050x = 1400
now its as simple as solving for x
(1050x)/1050 = 1400/1050
simplify
x = 4/3 convert to a mixed fraction because we need the time.
1 1/3 what is 1/3 of an hour 1/3 * 20/20= 20/60 so.. 20 minutes and an hour.
the planes will be 1400 miles apart at 1:20 pm
how long will they be apart at 1400 miles has become much easier because we have our time x figured out as 4/3
600(x) or 600(4/3) = 800
450(x) or 450(4/3) = 600
this makes sense because if we add them together we get the distance 1400 our original distance. Hope this helps.