These three look like three topics but they are one idea: a percentage of a price, either taken off or added on. A discount of 15% leaves 85% of the usual price, so you can reach the price paid in one multiplication. GST in Singapore is 9%, added on, so the total payable is 109% of the ticket price. Simple interest is the same amount every year, always worked out on the money that was put in at the start. The commonest lost mark in this lesson is not arithmetic — it is answering the discount when the question asked for the price paid.
Worked example
Problem The usual price of a bicycle is $360. During a sale it is sold at a discount of 15%. How much does Ravi pay for the bicycle?
The usual price $360 is the whole, so it counts as 100%.
A 15% discount leaves 100% − 15% = 85% of the usual price.
85% of $360 is 17/20 × 360 = 306, so Ravi pays $306.
Watch out The discount here is $54. That is a real number in this question, but it is not the answer — read the last line again before you write anything down.