I'm using Ubuntu 12.04 (precise) 64-bit, My system configuration is Memory: 7.8 GB and Processor :Intel® Core™ i 7-2600 K CPU @ 3.40 GHz × 8 , in system monitor resource page it shows using 7.4 GB RAM but overall process in process page uses around 3 GB RAM, how to find the which process using the reaming RAM and how to solve this problem.
7.4 GB of memory is quite much. The system is probably allocating memory for buffers, and not all that memory is actually used.
You can inspect memory usage by running top and then pressing M (to sort processes for memory usage).