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You want to find the area of (border+pool) and then subtract the pool.

(border+pool) has dimensions 35 by 15 (25+5+5 by 10+2.5+2.5).

That area is 525.

The pool area is 25 by 10, or 250

525 - 250 = 275 m^2

Well your pool is 25m by 10m as a rule the long sides the sides and the short sides are the ends. So you have 2 sides at 25m each and 2 ends at 10m each.

the area of the boarder is 2(the number of sides) X 25(the length of the side) X 2.5 (the width of the boarder = 2 x 25 x 2.5 = 50 x 2.5 = 125 square metres

Similarly the ends are 2 x 10 x 5 = 20 x 5 = 100 square metres

The question asks how many square metres of boarder is required. So 125 square metres + 100 square metres is 225 square metres.

Therefore 225 square metres are required for the pool.

if the sides are 25m you twice the width of the concrete wall to each to give you an outside dimension of 15m x 35m

the area of the whole thing would be 525m^2. now subtract the area of the pool which is 250m^2 to get 275m^2 of concrete. if the sides are the 10m the outside dimensions would be 30 x 20 for an area of 600m^2. subtract the 250m^2 for the pool to get 350m^2 for the concrete