Final answer:
Sienna's error in using base-ten blocks to find 45 divided by 3 is explained.
Step-by-step explanation:
Sienna used base-ten blocks to find 45 divided by 3. The error in Sienna's approach is that base-ten blocks are used to represent place value and not to directly divide numbers. To find the division, Sienna should have represented 45 with base-ten blocks and then grouped them into sets of 3 blocks to see how many sets can be formed. By doing this, Sienna would have found that 45 divided by 3 is 15.