A swimming pool is 20 m long 15 m wide and 3 m deep.

Length of the swimming pool $=20 \mathrm{~m}$

Breadth $=15 \mathrm{~m}$

Height $=3 \mathrm{~m}$

Now, surface area of the floor and all four walls of the pool $=($ length $\times$ breadth $)+2 \times($ breadth $\times$ height $+$ length $\times$ height $)$

$=(20 \times 15)+2 \times(15 \times 3+20 \times 3)$

$=300+2 \times(45+60)$

$=300+210$

$=510 \mathrm{~m}^{2}$

The cost of repairing the floor and the walls is Rs $25 / \mathrm{m}^{2}$.

$\therefore$ The total cost of repairing $510 \mathrm{~m}^{2}$ area $=510 \times 25=\mathrm{Rs} 12750$