A rectangular water reservoir contains 105 m

Length of the rectangular water reservoir $=12 \mathrm{~m}$

Breadth $=3.5 \mathrm{~m}$

Suppose that the height of the reservoir $=h \mathrm{~m}$

Also, it contains $105 \mathrm{~m}^{3}$ of water, i.e., its volume $=105 \mathrm{~m}^{3}$

Volume of the cuboidal water reservoir $=$ length $\times$ breadth $\times$ height

$\Rightarrow 105=12 \times 3.5 \times h$

$\Rightarrow 105=42 \times h$

$\Rightarrow h=\frac{105}{42}=2.5 \mathrm{~m}$

$\therefore$ The depth of the water in the reservoir is $2.5 \mathrm{~m}$.