In a parallelogram, opposite angles are equal, and consecutive (or adjacent) angles are supplementary, meaning their measures add up to 180°. This is because the internal angles of any parallelogram must satisfy this condition.

Given that $\angle A = 99^\circ$, the adjacent angle $\angle B$ must satisfy the following equation:

$\angle A + \angle B = 180^\circ$

Substituting the value of $\angle A$:

$99^\circ + \angle B = 180^\circ$

Now, solving for $\angle B$:

$\angle B = 180^\circ - 99^\circ = 81^\circ$

Thus, the measure of $\angle B$ is $81^\circ$ because adjacent angles in a parallelogram are supplementary.

Suggested Response:

"The measure of $\angle B$ must be $81^\circ$ because in a parallelogram, adjacent angles are supplementary. Since $\angle A = 99^\circ$, we subtract it from $180^\circ$ to find $\angle B = 81^\circ$."

Would you like any further details or clarifications on this?

Here are 5 related questions to deepen your understanding:

1. How do you prove that adjacent angles in a parallelogram are supplementary?
2. What are the properties of a parallelogram regarding angles and sides?
3. If $\angle B$ were given instead of $\angle A$, how would you calculate $\angle A$?
4. What happens if the parallelogram is a rectangle? What would the angles be?
5. How would the problem change if the shape were a trapezoid instead of a parallelogram?

Tip: Always remember, in any quadrilateral, the sum of all interior angles is $360^\circ$.