Answer:
400 yards.
Explanation:
Since it is a right-angled triangle, you will need to use Pythagoras theorem:
a² + b² = c² (with c being the hypotenuse, the longest side, and a and b being the other two sides).
a² + 300² = 500²
a² + 90000 = 250000
a² = 160000
a = 400
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You can also utilise Pythagorean triplets here in this circumstance. 3,4,5 are a very common one. This means that powers of 10 of this triplet will also be a triplet. So if you have 300 and 500, you know that it is still part of this triplet and 400 hundred will have to be there. You could simplify the numbers beforehand and then multiply them by 100 at the end if it helps.
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Hope this helps! :)