groups of 7 equals 21 \( \times 7=21 \) threes equals 21 \( +3+3+3+3+3+3=21 \)

Did you know that the number 21 is fascinating in mathematics? It’s known as a triangular number, meaning it can be arranged in an equilateral triangle. If you were to visualize it, you would see 6 dots in the first row, 5 in the next, and so on, culminating in 1 dot at the top. And there you have it, the total is 21! In real-world scenarios, the concept of grouping numbers can help in simplifying tasks. For example, if you’re arranging chairs for a party and you know each table can seat 7 people, you'll quickly figure out you need 3 tables for 21 guests. Using the idea of multiples makes planning events much more straightforward and organized!