The dimensions of a rectangular water tank are 2 m 75 cm by 1 m 80 cm by 1 m 40 cm.

$1 m=100 \mathrm{~cm}$

Therefore, dimensions of the tank are:

$2 m 75 \mathrm{~cm} \times 1 m 80 \mathrm{~cm} \times 1 m 40 \mathrm{~cm}=275 \mathrm{~cm} \times 180 \mathrm{~cm} \times 140 \mathrm{~cm}$

$\therefore$ Volume = Length $\times$ Breadth $\times$ Height $=275 \times 180 \times 140=6930000 \mathrm{~cm}^{3}$

Also, $1000 \mathrm{~cm}^{3}=1 L$

$\therefore$ Volume $=\frac{6930000}{1000}=6930 L$