The dominating factor responsible for the decreasing ionisation energies of the elements on moving down the group is `:`

Answer

4

Explanation

Let mole fraction of `O_(2)` is `x`
`40=32xx x+80(1-x)" "`or `" " x=5//6`
`a+b=x:(1-x)=(5)/(6):(1)/(6)`
When ratio is changed
`M_("mixture")=32xx(1)/(6)+80xx(1)/(6)=72`