Final answer:
Brianna used 4 pencils out of the 10 she started with.
Step-by-step explanation:
To determine how many pencils Brianna used, we can multiply the fraction representing the number of pencils she used by the total number of pencils she started with.
The fraction representing the number of pencils she used is 2/5, so we can calculate: (2/5) * 10 = 4.
Therefore, Brianna used 4 pencils.