Given,

ABCD is a rhombus

∠ACB = 40°

∵ ∠DAC =∠ACB = 40° [alternate angles]

∠AOD = 90° [ diagonals of rhombus are perpendicular to each other]

In ∆AOD

∠AOD+∠ADO+∠DAO = 180°

∠ADO+90°+40° = 180°

= ∠ADO = 180°- 130° = 50°

=∠ADO=∠ADB = 50°