Capital Gains Tax for a widow

My parents bought a house that they owned jointly. When my father died, my mother inherited his share. She has now sold the house. Because she has been in a care home for more than three years, there will be CGT to pay, but how do I calculate the amount? I could do one calculation for the 50% that she has always owned, and a second calculation for the 50% that she inherited more recently? Sorry if the answer is obvious, but I couldn't find it

Hi DCO9753,
You take the 50% share of the value when purchased and the other 50% of the value at the time your father passed.
Added together this is the purchase price.
Thank you.