hi. im a malaysian. i have a multiple china visa which expire on 8th july. im enter china on 8th may

im enter china on 8th may. each stay is 30 days. can i go to hong kong n reenter china the same day before my 30 days expire?? which on 7th june? and with my vis expire on july8, am i eligable to stay in china till 8th august??