Surface Area and Volume · Superchallenge problem

A fish tank is a box $60$ cm long, $40$ cm wide and $50$ cm tall, and it holds water $29$ cm deep. Solid bricks, each $10$ cm by $10$ cm by $20$ cm, are lowered in one at a time and sink to the bottom. What is the greatest number of bricks that can be put in before any water spills over the top?