Kristen has 7.5 mL of water and her lab partner Rebecca has 2.3 L. How many mL do they have?

Q:
Kristen has 7.5 mL of water. Her lab partner Rebecca has 2.3 L of water. How many mL of water do they have between the two of them?
A: 
The key to this question is the knowledge that one liter (L) of water (or anything, for that matter) is equivalent to 1,000 milliliters (mL). An easy way to remember this is that the prefix "milli-" always means one-thousandth. Since the liter is the standard unit, a milliliter is one-thousandth of a liter. In other words, a liter is made up of one thousand milliliters.
In order to add the two together, we need them to be in the same units. Since the question is asking us for milliliters, let's convert Rebecca's water into milliliters, as well (we already know Kristen has 7.5 mL). Converting will give us a common unit that we can add together. 
Since we know there are 1,000 milliliters (mL) in a liter (L), we can use multiplication to get the number of milliliters that Rebecca has. In other words, Rebecca has:
2.3 L * (1,000 mL)/L = 2,300 mL 
You can read the above as "2.3 liters times 1,000 milliliters per liter." Since we are multiplying, the liters unit ("L") from "1,000 mL/L" is cancelled out and we are left with the number of milliliters. Now we just add the two values together to get the total number of milliliters that Kristen and Rebecca have:
2,300 mL + 7.5 mL = 2,307.5 mL
So in total, they have 2,307.5 mL of water, which is the final answer.
https://www.britannica.com/science/litre