Question: my car goes $8$ km on a litre of petrol. i use $18$ litres on a certain trip. how much petrol should i have saved if i went $9$ km?
1. **State the problem:**
You have a car that runs $8$ km per litre of petrol. On a trip, you used $18$ litres. You want to find how much petrol you would have saved if you had driven $9$ km instead of the original distance.
2. **Find the original distance traveled:**
The original distance $D$ is given by the formula:
$$D = \text{fuel used} \times \text{distance per litre}$$
So,
$$D = 18 \times 8 = 144 \text{ km}$$
3. **Calculate petrol used for $9$ km:**
To find petrol used for $9$ km, use:
$$\text{petrol used} = \frac{\text{distance}}{\text{distance per litre}} = \frac{9}{8} = 1.125 \text{ litres}$$
4. **Calculate petrol saved:**
Petrol saved is the difference between original petrol used and petrol used for $9$ km:
$$\text{petrol saved} = 18 - 1.125 = 16.875 \text{ litres}$$
5. **Answer:**
You should have saved $16.875$ litres of petrol if you had only gone $9$ km.