1. The problem asks to draw a tally for the number 21. 2. Tally marks are grouped in sets of five, where four vertical lines are followed by a diagonal line crossing through them, representing 5. 3. To represent 21 using tally marks, count how many groups of 5 fit into 21: $$\left\lfloor \frac{21}{5} \right\rfloor = 4\text{ groups of 5}$$. 4. The remainder after grouping fours sets of 5 is: $$21 - 4 \times 5 = 1$$. 5. So, 21 tally marks are represented by 4 groups of 5 (each group has 5 tally marks) plus 1 single tally mark. 6. In tally notation, that would look like: 4 groups of Ⅼ and one single |. 7. Therefore, the tally for 21 is |||| \ (four sets of five) and | (one single).