Question

if ∠abc and ∠def are vertical angles,
and m∠abc is equal to 90°, then what
is m∠def?
m∠def = ?°

Explanation:

Step1: Recall the property of vertical angles

Vertical angles are equal.

Step2: Apply the property

Since $\angle ABC$ and $\angle DEF$ are vertical angles and $m\angle ABC = 90^{\circ}$, then $m\angle DEF=m\angle ABC$. So $m\angle DEF = 90^{\circ}$.

Answer:

$90$