Windows require at least 20GB space on HDD and I can see you have 20,87GB free which is not enough. It is because Windows see partition size differently so if os x shows 20 windows may see less. And bootcamp probably takes this into consideration.
How to partition?using disk utility can't extend more than 8gb
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Downloading windows support software running more than 10days but still 30% so I downloaded win support software from direct link.all I want is to make the greyed out install windows 7 check able.how to do that brother?