Question 1141751
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Since the pentagon is regular, its side length is  {{{1/5}}}  of its perimeter, i.e. 3 mm.


Having the hypotenuse of 5 mm, the given right angled triangle is  (3-4-5) triangle; hence, its other leg is 4 mm.


Then the area of this triangle is  {{{(1/2)*(4*3)}}} = 6 square millimeters.


<U>ANSWER</U>.  The answer is B).
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