Answer
The correct answer is $\frac{6}{7}$.
Work Step by Step
The student's error is in the step where he attempted to reduce the fractions by the common factor 3. The denominator of $\frac{20}{21}$ should have been changed to 7 ($21 \div 3$), not 3.
This would have resulted in the following equation:
$\frac{9}{10} \times \frac{20}{21}$ = $\frac{3}{1} \times \frac{2}{7}$ = $\frac{6}{7}$
