Final answer:
Elliot has added 3 ml of pure milk fat to his coffee.
Step-by-step explanation:
To find the amount of pure milk fat that Elliot has added to his coffee, we need to calculate 20% of 15 ml.
- Convert 20% to a decimal by dividing by 100. 20% = 0.2.
- Multiply 0.2 by 15 ml. 0.2 * 15 ml = 3 ml.
Therefore, Elliot has added 3 ml of pure milk fat to his coffee.