Partial charges question: 30 days vs 31 days

[wireless]

Wirelessly posted (iPhone 4: Mozilla/5.0 (iPhone; CPU iPhone OS 5_1_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9B206 Safari/7534.48.3) Lets say my billing cycle is on 15 of every month. I added a $10 international text messaging plan on 25th. How much will partial...

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