a) in 1/60 seconds 1 track is read, so in 1/60 seconds 512 sectors are read, so 4 sectors are read in = 4/(60*512) sec.
b) in 1/60 seconds 1 track is read, so in 1/60 seconds 512 sectors are read, each sector 512 kb, so in 1 second --->(2^18)/60 bytes.