Building the Number 21: Representing with Ones and Tens Blocks

To construct the number 21 using tens and ones, follow these steps:

• Step 1: Determine the number of tens: Divide 21 by 10, we get 2. There are 2 tens in 21.
• Step 2: Determine the number of ones: Subtract the total value of tens from 21. Calculate $21 - (2 \times 10) = 1$. This means there is 1 one.

Therefore, the number 21 is made up of 2 tens and 1 one. This is represented in the choice where there are exactly 2 units labeled as "tens," and 1 unit labeled as "ones." Comparing with the given choices, the representation in choice 2 correctly matches this breakdown.

Thus, the correct answer is choice 2.