Richa bought 6 bottles, each containing 400 ml of oil.

To find the total amount of oil she bought, multiply the volume of one bottle by the number of bottles:

$\text{Total oil} = 400 \, \text{ml} \times 6 = 2400 \, \text{ml}$

Thus, Richa bought 2400 ml of oil.

Would you like more details or further clarification?

Here are 5 related questions to consider:

1. If Richa used 150 ml of oil from each bottle, how much would she have left?
2. How many liters of oil did she buy if 1000 ml equals 1 liter?
3. If each bottle costs $5, how much did Richa spend on the 6 bottles?
4. How many bottles would she need to buy if she needed 3200 ml of oil?
5. If Richa used 300 ml of oil from one bottle, how much oil remains in that bottle?

Tip: Always convert units when needed, especially when dealing with milliliters and liters.