The external dimensions of a closed wooden box are 48 cm, 36 cm, 30 cm.

The outer dimensions of the closed wooden box are $48 \mathrm{~cm} \times 36 \mathrm{~cm} \times 30 \mathrm{~cm} .$

Also, the box is made of a $1.5 \mathrm{~cm}$ thick wood, so the inner dimensions of the box will be $(2 \times 1.5=3) \mathrm{cm}$ less.

i. e., the inner dimensions of the box are $45 \mathrm{~cm} \times 33 \mathrm{~cm} \times 27 \mathrm{~cm}$

$\therefore$ Volume of the box $=45 \times 33 \times 27=40095 \mathrm{~cm}^{3}$

Also, the dimensions of a brick are $6 \mathrm{~cm} \times 3 \mathrm{~cm} \times 0.75 \mathrm{~cm} .$

Volume of a brick $=6 \times 3 \times 0.75=13.5 \mathrm{~cm}^{3}$

$\therefore$ The number of bricks that can be put in the box $=\frac{40095}{13.5}=2970$