Capital Gains with 2 owners for 2 different lengths of time.

[quickquestion]

Hi

My parents are selling their second home and we are trying to work out the capital gains they will have to pay.

The strange thing about this is that the house was originally transferred into my Mums name in 1993 due to her Mums death so she has owned the property since then but in 2007 my Dad was added to the house and therefore they became joint owners.

How do we work out the capital gains to be paid if one has owned the property for longer?

[maths]

Mum inherited property at its then probate value i.e. market value (e.g. 250).

As it was not parent's main residence then in 2007 when i assume 50% was transferred to dad then dad effectively inherited at a cost of 50% of mum's original inherited cost (i.e. 5-0% of 200 namely 125).Mum's left with 50% at a cost of 125.

On sale each have a cgt charge each based on 50% of sale proceeds less (in the above example)m cost of 125.