Chapter 5: Comparing Quantities Using Proportion

Every topic in Comparing Quantities Using Proportion — profit and loss, discounts, GST, simple and compound interest — reduces to the same underlying idea: comparing one quantity to another as a ratio or percentage. It's also one of the most directly useful chapters in Class 8, since the same calculations show up in everyday shopping and banking, not just on paper.

A frequent error is applying a discount and GST in the wrong order, or applying GST to the original price instead of the already-discounted one — a Rs. 1,000 item with a 10% discount and 12% GST comes to Rs. 900 + 12% of Rs. 900 = Rs. 1,008, not 12% of the original Rs. 1,000. For compound interest specifically, forgetting that interest is calculated on the growing amount each year — not repeatedly on the original principal — is the single most common mistake.