|Your OP has 2 different rates listed.|
First you said: "a usage rate of 2.04 cents per MB"
Then you quoted: "Your current data rate is $0.02400 per Mb"
I'll assume the 2.4 cents ($0.024) is correct and use that number.
You also asked 2 different questions.
In you OP you asked: "if she got a balance of say $120.00 what does that convert to in MBs?"
In Response #2 you asked "In the example she has given ($209.00) how does that convert to gigabytes or a fraction thereof."
I'm not sure whether you want the answer in Mb or Gb, so I'll just toss out a bunch of stuff and let you decide what you can use.
In "computing" a Kilobyte is 1024 bytes.
Depending on who you ask, a Megabyte is either 1000 Kilobytes (1,024,000 bytes) or 1024^2 (1,048,576 bytes). To get an exact number, you would need to know which "definition" her ISP is using.
Depending on who you ask, a Gigabyte is either 1000 Megabytes (1,024,000,000 bytes) or 1024^3 (1,073,741,824 bytes). Again, to get an exact number, you would need to know which "definition" her ISP is using.
I'm going to take the easy way out and use 1000 Kb = 1 Mb and 1000 Mb = 1 Gb.
Since the terms of her agreement are "Your current data rate is $0.02400 per Mb charged per Kb or part (thereof)" we'll assume that usage is rounded up to the nearest Kb. In other words, if she uses 900 bytes, she'll be charged for a full Kb or 1,024 bytes.
So, at $0.024 per Mb,
50 / .024 = 2,083.333 Mb (that's your 2 Gb)
209 / .024 = 8,708.333 Mb
209 / .024 / 1000 = 8.708333333 Gb
We can check this by using our original "$50 for 2 Gb" numbers.
209 / 50 * 2,083.333 = 8,708.333 Mb
Now we have to deal with the "Kb or part thereof" issue.
8,708.333 Mb x 1000 = 8,708,333.3333 Kb
Since we can't use the .3333 Kb without paying for a full Kb, we'll ignore it. In other words, it can't be paid for out of the $209 so we have to "ROUNDDOWN" the answer if we want the balance shown in terms of how many Kb she can get for her $209.
OK, back to Excel...
Put 209 in A1.
In B1 use:
=$A$1/0.024/1000 for an approximate answer in Gb.
=$A$1/0.024 for an approximate answer in Mb
=ROUNDDOWN($A$1/0.024*1000,0) for an exact answer in Kb