You can't go by high speed train from Nanjing to Hefei, for the first speed train arrives in Hefei at 10:06am, and it is too late. The train earlier should be K8486 which will arrive in Hefei at 5:40am. No any other train between 5:40-10:06. It takes over 2 hours by bus. The first bus in Nanjing Zhongyangmen Bus Station leaves at 5:50. You can make it then. My suggestion: take the ealiest bus or get to Hefei one day in advance.