For a reaction R1 , For a reaction R2 , Reaction R1 is non-spontaneous but along with R2 it is spontaneous. This means that

A process is non spontaneous means ∆G=+ve ;Thus for R1, ∆G=x=+ve; But for R2 ∆G=y=-ve because it is spontaneous; R1 becomes spontaneous in presence of R2, this is possible only when magnitude of y>x ; Such reactions are known as coupling reactions.