Because this topic is that two people start at the same time and go in the opposite direction, which does not involve personal speed, but the sum of their speeds is constant.
Only the total speed (moving in opposite directions, adding the two speeds) is constant (! ! ) this condition into the equation.
Let AB be x kilometers apart, then:
(x-36)/( 10-8)=(x+36)/( 12-8)。
Where (x-36) means two people at 8 am->; The total distance moved by the point 10( 10-8),
(x-36)/( 10-8)- is the total speed of two people in (10-8) time.
(x+36) is the total distance traveled by two people from 8 am to 12.
(X+36)/( 12-8) is the total speed of two people, that is, the sum of their speeds.
From beginning to end, the total speed of the two people remained the same, so the above equation was established.
If you still don't understand, please ask.