1. A block is a grouping of contiguous bytes on disk which are transferred together to RAM. It is needed as the atom of data transfer because the access-transfer ratio is >>1. If it is too small, too much time will be pent seeking. If it is too big, it will not fit within a track length (or possibly even into RAM). A rule of thumb to determine the size of a block is make it equal to the access-transfer ratio: in this case, about half the time will be spent seeking, half transferring.
  2. t= 10^6/(b*(s/60)), l= 10^3/(2*(s/60))