A boat carrying steel balls is floating on the surface of water in a tank. If the balls are thrown into the tank one by one, how will it affect the level of water 

${\rho }_w<{\rho }_{steel}$

when steel balls are in boat, to balance it equal weight of water displaced. This volume of water displaced is large compared to the volume of water displaced when the steel balls are dropped in to water. Hence water level falls.