An exterior angle

Let, ∠1 and ∠2 be two opposite interior angles and ∠3 be exterior angle.

According to question,

∠1 + ∠2 = ∠3

∠1 + ∠1 = 100^o

2∠1 = 100^o

∠1 = 50^o

Therefore, ∠1 = ∠2 = 50^o

Thus, of these angles is equal to 50^o.