Discount First, Then GST
Why the order matters: calculating the final price of a ₹5000 phone.
Equation
y = x*(1-5/100)*(1+5/100)
Graph
Table
| x | y |
|---|---|
| 0 | 0 |
| 500 | 498.75 |
| 1000 | 997.5 |
| 1500 | 1496.25 |
| 2000 | 1995 |
| 2500 | 2493.75 |
| 3000 | 2992.5 |
| 3500 | 3491.25 |
| 4000 | 3990 |
| 4500 | 4488.75 |
| 5000 | 4987.5 |
What this lesson covers
What you do
You shape the function y = x*(1-d/100)*(1+g/100) and watch the graph answer.
Challenges to clear
- Set discount d = 10% and GST g = 18%. The ₹5000 phone lands at ₹5310 — find the x = 5000 row.
- Same rates, cheaper phone: ₹2000 becomes ₹2124.
- A bigger sale: slide the discount d to 20%.
- Now find the GST rate that makes a Rs 2500 item ring up at Rs 2240. Discount first, tax second — the order matters.
Check yourself
If a shopkeeper offers a 20% discount and charges 10% GST, why can't you just say the net change is -10%?
- Because the discount is taken off the original price, but GST is added to the already-reduced price. The bases are different. — correct
- Because GST is always higher than the discount in India.
- Because you have to subtract the discount from the GST percentage first.
- Because the shopkeeper makes a profit on top of GST.
Think about it
- ₹5000 phone: 10% discount first (₹4500), then 18% GST on the discounted price. Final price?