I will assume that the ends the sides that are 25 m apart...
A=(25+2*5)(10+2*2.5)-25*10
A=(35)(15)-250
A=525-250
A=275 m^2
If it was the other way around and the wider border was along the long sides...
A=(10+2*5)(25+2*2.5)-25*10
A=(20)(30)-250
A=600-250
A=350 m^2
