Question 1034618
Not too tough to draw a reasonable diagram on paper.  


The pony can go into three regions:


A sector of 270 degrees of radius 50 which gives  {{{(270/360)*pi*50^2}}} square feet;  this is  {{{(3/4)pi*2500}}};


Smaller sector of a circle of 90 degrees and radius 10 feet giving {{{(1/4)pi*10^2}}} square feet;


Medium sector of a circle of 90 degree, radius 30 feet to give {{{(1/4)pi*30^2}}};


The SUM of these area regions.



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The pony can stretch the tether line 50 feet along one wall of 40 feet, leaving 10 feet more to stretch along the next wall.
The ponly can stretch the tether line 50 feet along the other wall of 20 feet leaving 30 more feet to stretch along the next wall.