Question 335496
let speed of cargo plane be x km/h
jet plane = 1000 km/h
..
after 6 hours they had traveled 
cargo plane 6x km
jet 6000 km
..
6000-6x= 2100
add +6x to both sides
600 +6x-6x = 2100 +6x
6000 = 2100 +6x
add -2100
6000-2100=6x
3900 =6x
divide by 6
x= 650 km/h speed of cargo plane