An external computer flash drive can hold 1 gigabyte of data. how many bytes is this

There is quite a bit of confusion in the industry.
In computer science, most people loosely consider 1 Gb as 2^30=1073741824 bytes, while the prefix Giga actually means 10^9, or 1,000,000,000.

When we buy a hard disk, we are told (correctly) that it holds 1 Gb.  Manufacturers base it on the mathematical definition of Giga and give you 10^9 bytes (less than 1073741824=1Gibibyte, or 1 GiB).  This is standard practice in the disk drive industry.  For example, a Windows 7 will show a 3 tera byte external disk as having 2.72 TB, or 3,000,557,891 byes, in which case it is actually assuming a TB to mean 2^40 byes (instead of TiB).  That's where confusion lies.

Can something be done about it?  Yes, it has been done since 2007.
The less well-known, but official quantity for 1073741824 bytes is one GiB, proposed by the International Electrotechnical Commission (IEC), adopted by IEEE, EU, and NIST.  Similarly, 1 TiB means 2^40=1,099,511,627,776 bytes.  The prefixes have been replaced by bi (for binary), so Kibibyte, Mebibyte, Gibibyte, Tebibyte, etc.are used to represent 2^10, 2^20, 2^30, 2^40 bytes.  Unfortunately I have yet to see this widely being used, or even known!

$100. 125 x 0.20 = 25125 - 25 = $100