At noon a private plane left Austin for Los Angelas, 2100 km away, flying at 500 km/h. One hour later a jet left Los Angelas for Austin at 700 km/h. At what time do they pass each other?

The distance covered by the private plane is given by:

d_1 = 500t

The distance covered by the jet is given by:

d_2 = 700(t-1)

Since it departed 1 hour later (at t = 1, it hasn't covered any distance).

The sum of both distances now give a total of 2100 km, if they meet (the part covered by the jet and the part covered by the private plane equal the total distance).

d_1 + d_2 = 2100

Just substitute the equations in there and find t, the time they took to meet each other. Then add that t in hours to noon to get the time they meet.