Child product discount

[squawk]

Hi
Does anyone know how I can apply a discount to a child product if the parent (which is the first item within the show child prdouct list) product is selected.
ie If there is a parent product of say £20 and a child product of say £15 the child product becomes £10 when the customer selects the £20 parent item.
Such as when a customer but a hard copy of a manual the eletronic version becomes cheaper as to encourage the customer to but a downloadable version as well.

Can anyone help?

Many Thanks

[squawk]

Has anyone got any suggestions on this???