a pool is 50 meter long, 25m wide and 4m deep.what will be the cost of cementing its walls and the floor at at a rate of 25 per meter?

The price is more likely to be per SQUARE meter. You have not said what the monetary units are. Euros? Rupees? Renmibi?

I suspect they want to to compute the total area of the sides and bottom of the pool, and multiply by the cost per area.