How many bytes is 20 megabytes?

1 Bit is the smallest recordable state ( Zero or One / ON or Off / High or Low )

1 Byte is 8 Bits which can represent a number upto 256 ( 2**8) or a character

1 KiloBytes (KB) is 2**10 Bytes == 1024 Bytes //// "Slang" as One Thousand

1 MegaBytes (MB) is 2**20 Bytes = 2**10 * 2**10 Bytes == 1024 * 1024 Bytes == 1,045,876 Bytes //// "Slang" as One Million

1 GigaByte (GB) is 2**30 == 1,073,741,824 //// "Slang" as One Billion

So 20 Gigabytes is 20 * 2**30 Bytes

1 TeraByte (GB) is 2**40 == 1,099,511,627,776 //// "Slang" as One Trillion

From the above, you can see why a 32 bit operating system can address upto 4 GB of memory (2**32 == 2**2 * 2**30 == 4 * 1 GB == 4GB

if your computer has 4GB of RAM and a 32 bit OS, it will not be able to address it all since some of that address is used by video cards and other devices.