Interest charge on installment payment

I saw the following advertisements in the newspaper:

Acer netbook, cash price $599, weekly installment of $5.47 over 48 months
Toshiba notebook, cash price $939, weekly installment of $8.58 over 48 months

In both cases, the installment payment plan charges an interest rate of 0.7% per week, or 36% per annum. By paying cash, the consumer will save on interest rate of 36%.  The total amount payable over 4 years represent an additional 90% of the cash price.

If the consumer pays in cash, he would be able to get two netbooks or notebooks for the same amount payable.

Note: you can calculate the interest rate in this type of transaction by using the Excel function called Rate.

Tan Kin Lian