So we can just subtract the area of the pond from the area of the whole space.
So to find the area of the pond,
200²
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.
That would equal about 125663.7 ish.
And then the whole area would be
250²
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.
That would equal about 196349.541
And then just do
196349.541 - 125663.7 = 70685.741 ish
So they would need about 70685.741 square feet of gravel.