Data Sufficiency

Sally has a gold credit card with a certain spending limit, and a platinum card with twice the spending limit of the gold card. Currently, she has a balance on her gold card that is 1/3 of the spending limit on that card, and she has a balance on her platinum card that is 1/5 of the spending limit on that card. If Sally transfers the entire balance on her gold card to her platinum card, what portion of her limit on the platinum card will remain unspent?

A) 11/30
B) 29/60
C) 17/30
D) 19/30
E) 11/15

My Approach:

Say the gold card has spending limit x,
So the platinum card has spending limit 2x

Balance on gold card = x/3
Balance on platinum card = 2x/5

Sally transfers the entire balance on her gold card to her platinum card: So balance on platinum card = 2x/5 +x/3 = 11x/15.

Now, doesn't balance means the unspent amount?
If that is so, then the answer should be E. But that is an incorrect answer.
What does "balance" here mean exactly?

[spoiler]OA: D[/spoiler]

I think the easiest way here is to use artificial numbers, because whatever you pick, the fraction will be the same. Make sure you pick numbers that are multiples of the fractions provided in the question. Instead of using variables that get hard, just pick easy numbers.

I picked:
Gold Limit: 15
Plat Limit: 30

Gold Balance: 1/3*15=5
Plat Balance: 1/5*30=6

Transfer divided by limit on plat: 5+6 / 30 = 11/30

Unspent = Limit - balance = 30 - 11 = 19

19/30

"Balance" here means amount charged to the card.

Ok!

If that is wha unspent amount means, then answer should be (2x - 11x/15)/2x = 1 - 11/30 = 19/30!

Now I got it!