You must delete the RedHat Linux partitions before installing Ubuntu.
Press 'Alt + F2, this will open the 'Run Application' dialog, type 'gksu gparted'
(without quotes). Now delete the redHat Linux partitions using gparted.
Leave 10 GB unallocated space in the partiton. During Ubuntu partitioning setup,
choose, 'Allocate from the free space available) ie., option 2. It will automatically
allocate the space for root & swap.Or if you want to allocate manually, allocate
8 GB for root ('/') & 1.5 GB for swap.