Click here to rea my article CC are Cash Cow for Banks. You may just find the answer you are looking for.

Since you do not have a credit card yet, all you need to know is that the interest rate imposed on outstanding balance (if you don't pay in full before Due Date) for new cardholders is 17.5%.

Since you have yet to get your card, I will share with you this, from the very start, never ever let the bank imposed interest on your outstanding balance no matter how small or little the amount is, i.e. always pay in full before Due Date. Once you break this principal, you have entered into the hell zone.

If you pay in full every month, it does not matter if the banks imposed 88% or 4% interest on outstanding balance.

But is you really must know how the 13.5% -17.55 is charged, go to any bank website and read the Terms and Conditions for credit cards and you'll get the answer.
Not that I know. The 5% for petrol is for 1st weekend of the month and other days 2%. But, there are terms and conditions and the the cash back is based on 20% of total retail transactions. What I interpret from this is - if you only use the card purely for petrol it means you won't get any cash back.

When you first get the card the interest rate is 17.5% per annum

as long as you pay ontime for 10 months continuously the interest rate goes down to 16% per annum

pay on time for 12 months and more continuously the interest rate goes down to 13.5% per annum.

Miss one payment or make 1 late payment and interest rate resets to 17.5% per annum and you start from scratch again