Y! market cap is $36.38b. Have in mind that Y! is selling only its core biz so we would have to subtract values of Y! shares in Alibaba and Y!Japan, which are worth $33.74b and $8.56b respectively. However thats pre-tax and Y! could not get that money for them. There for adjusted values (-38% tax)are $21b and $5.4b again respectively. There are also cash & marketable securities worth $6.8b and convertible debt of $1.4b.
So final math looks like this: $36.38b-$21b-$5.4b-$6.38b-(-$1.4b)=$5b