1. **State the problem:** Multiply 21 by \frac{5}{7}. 2. **Formula used:** To multiply a whole number by a fraction, multiply the whole number by the numerator and then divide by the denominator. 3. **Intermediate work:** $$21 \times \frac{5}{7} = \frac{21 \times 5}{7}$$ 4. **Simplify the fraction:** $$\frac{21 \times 5}{7} = \frac{\cancel{21}^3 \times 5}{\cancel{7}^1} = 3 \times 5$$ 5. **Final calculation:** $$3 \times 5 = 15$$ 6. **Answer:** The product of 21 and \frac{5}{7} is 15.