To estimate \( 28 \times 0.64 \), we can round 0.64 to a simpler number.
0.64 is close to 0.7. So, we can estimate:
\[ 28 \times 0.7 \approx 28 \times \frac{7}{10} = \frac{28 \times 7}{10} = \frac{196}{10} = 19.6 \]
Now, looking at your options (10, 15, 21, 28), the best estimate of \( 28 \times 0.64 \) would be 21.