Q: extra sapce as other in iPad

my iPad Air 2 , with 16 GIG hard memory , the newest IOS 9.3.2 , I missed nealry 5 GIG space and cannot revert this space even cleaning all files , apps , Photos ,Video . As I search in internet and do 6 suggested option and even restore but no changing at all .in Andorid is no problem like this and even by my notebook can delete files easily but IOS system is very compilcated and nearly all users complain about this problem . What is suggestion of you to help me . thanks