Okay, here we have this:
Considering the provided information, that in a rhombus opposite angles are equal, and that the sum of the angles of a triangle is 360 °, we obtain:
360°=100°+100°+4(m∠DBC)
Now, let's clear "m∠DBC":
360°=200°+4(m∠DBC)
4(m∠DBC)=360°-200°
4(m∠DBC)=160°
m∠DBC=160°/4
m∠DBC=40°
Finally we obtain that the correct answer is the option A.
