(days of primary residence) / (days of ownership)

1. eligible for capital gain exemption:  2 years of primary residence in the past 5 years of ownership.

Example A:  owned 2006-2015 (10), primary residence (2006-2007) (2); rented or second home 2008-2013 (6), primary residence (2014-2015) (2), you are eligible for capital gain exemption

But you might not be eligible for all of the gains, because: 

2.  How much capital gains are exempted: 

gains x (prmary residence) / ownership = exempted gains. 

same example:  

Your exepmted gains= 4/10 of the total gains 

You need to pay CGT on 6/10 of the gains.