∡b is a vertical angle to ∡a and therefore a twin, and we know that ∡a = 120°, thus since ∡b = ∡a, ∡b = 120° as well.
a full circle has a total of 360°, ∡d and ∡c are vertical angles, namely two angles across from each other at a junction, and therefore ∡d = ∡c.
∡a + ∡b is 120° + 120° thus 240°, since a circle has a total of 360°, 360 - 240 = 120, so the other two angles pick up that slack and divide it among each other evenly, 120/2 = 60, so ∡d = ∡c = 60°.
