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If you draw a diagram showing a vertical cross-section of the frustum, through the centre of the square, parallel to one of its sides, you'll see that the sloping face of the cone forms an isosceles triangle with the base and a vertical height. Since the height is 10m, the part of the base beneath the sloping face is also 10m. You should be able to see that the radius of the top surface of the frustum is therefore 25 - 10 = 15m.
Can you complete the calculation now?