Kesme and Soba the inheritance tax question

I need help on a question of BPP (UK tax) its question 18 Kesme and Soba the inheritance tax part…i understood the concept of spouse exemption..325,000 available NRB for gift to the daughter which was taxable..why the remaining 45,000 was multiplied 40/60 to give 30k as liability..can someone please explain ??.. i know tax rate on death is 40% then why divide by 60???