Hello everyone,

In relation to GRE Quant Concept Series focusing on work rate distance, I struggle in understanding why in some specific situations rates have to be combined or substracted.

In the practice video of a problem :

1/Greg explained the illustration where two trains go in opposite direction, why should we combine the rates ?

2/In the illustration when train has to catch up with another train heading the same direction, why should we subtract the rates ? wouldnt this be the same objective, therefore combining the rates ?

Thank you for your help