Let D be the number of bricks delivered by day d. Then

D=6400+1600(d-1),

or:

D=4800+1600d.

Let L be the number of bricks laid, then:

L=2400d

The brickies run out of bricks when D<=L, so we look for d such that:

4800+1600d=2400d.

Rearrange this to get:

800d=4800,

or d=6.

That is the brickies will have no bricks on hand at the end of day 6.

RonL