There is an error in your textbook 
(S&G, 4th edition plus some printings of S&G, 5th edition).
It is corrected in the 6th edition, p. 431.


What it says: (5th edition, p. 383 near bottom)

"A 1.3-gigabyte disk with 512-byte blocks would need a bit 
map of over 310K to track its free blocks."

It should say:

"A 1.3-gigabyte disk with 512-byte blocks would need a bit 
map of over 332Kb to track its free blocks."

Reasoning:

Disk Size:  1.3 G bytes = 1.3 * 2^30 bytes

Block Size:  512 bytes = 2^9 bytes

# of Blocks:  
= disk size / block size 
= 1.3 * 2^30 bytes / 2^9 bytes =  1.3 * 2^21

# of entries in bit table 
= # of blocks 
= 1.3 * 2^21

Size of bit table in bits 
= 1.3 * 2^21 bits

Size of bit table in bytes 
= size in bits / (8 bits/byte) 
= 1.3 * 2^21 / 8 bytes
= 1.3 * 2^21 / 2^3 bytes
= 1.3 * 2^18  bytes
= 1.3 * 2^8   * 2^10 bytes
= 1.3 * 256   * K bytes
= 332.8         K bytes
= 332.8K bytes