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Answer. Dimensions are 30 yards by 40 yards.
Classic (3,4,5)-right angled triangle.
Algebra solution
x^2 + (x+10)^2 = 50^2
x^2 + x^2 + 20x + 100 = 2500
2x^2 + 20x + (100-2500) = 0
2x^2 + 20x - 2400 = 0
x^2 + 10x - 1200 = 0
(x-30)*(x+40) = 0
The only positive root x= 30 is meaninful, leading to the answer.
Solved.