In rhombus ABCD

∠ACB = 40° given

∠ACB = ∠CAD = 40° [Altermate interior angles are equal]

In ΔAOD

∠AOD = 90° [In rhombus diagonals bisect eact other at right angles]

∠AOD + ∠CAD +∠ADB = 180° [Angle sum property of a triangle]

90° + 40° + ∠ADB = 180°

∠ADB = 180°-130°

∠ADB = 50°