This question is not a very good one because it requires some contextual knowledge of how billing works in Singapore. It is unlikely a question in the PSLE will require you to understand this following fact. Usually, discounts are applied to the original price, followed by the service charge and lastly the GST. This sequence is important for getting the right answer. Meaning, that you cannot simply add 10% and 7% to get 17% as the added cost because the service charge is also taxed (compounded). I hope it was sufficiently clear and helpful. Feel free to clarify and let me know if I made any errors.