Question

Two cars leave Calgary at the same time travelling in opposite directions. Their average speeds differ by 5km/h they are 210 km apart find the speed of each car

Answer 1

since distance = speed * time, if the slower car has speed s, then their combined speed is 2s+5
So if t hours later, they are 210 km apart, you have
t(2s+5) = 210

when you decide on t, you can determine the speeds

Answer 2

speed of slower --- x km/h
speed of faster = x+5 km/h

When they are 210 km apart, they both went for t hours
tx + t(x+5) = 210

leaving you with 2 unknowns but only one equation, not enough data
to get a unique answer.

e.g. let t = 2, then
2x + 2(x+5) = 210
4x = 200
x = 50, so one went 50 km/h the other 55 km/h

let t = 1.5
1.5x + 1.5(x + 5) = 210
3x = 202.5
x = 67.5, so one went 67.5 km/h the other 72.5 km/h
check: 1.5(67.5) + 1.5(72.5) = 210
so my answer is correct for this case.

you can verify my first case in the same way, it is also valid.