Final answer:
The builder used 35 ounces of plaster.
Step-by-step explanation:
In this question, you are given that a builder had 350 ounces of plaster and used 10% of it. To find the number of ounces of plaster used, you need to multiply 10% by 350. 10% of 350 is calculated by multiplying 350 by 0.10, which gives 35. Therefore, the builder used 35 ounces of plaster.