Answer:

Ammonia’s central atom (N) has 1 lone pair and 3 bond pairs.

Water’s central atom (O) has two lone pairs and two bond pairs.

The two lone pairs on the O- atom of water reject the two bond pairs. And the repulsion between lone pair and bond pair on H2O’s O-atom is higher than that on NH3’s N-atom.

Despite their distorted tetrahedral shape, H2O’s bond angle is shorter than NH3.