well there is a formula for area of the triangle and that is S*r....

In the given formula S is the semiperimeter i.e. half of the perimeter of the triangle. e.g. if a,b, and c are the sides of the triangle then perimeter will be a+b+c and semiperimeter will be (a+b+c)/2

Now, inradius is the radius of the circle that is inscribed in a triangle. In the given figure billow OP is an inradius.

Now, what all we know is three sides of the triangle, thus perimeter and area of triangle i.e. 60

Thus the easiest and fastest way is to apply the formula S*r = area of triangle

therefore, 20*r = 60 ...hence r = 3

Since r is the inradius i.e. radius of the inscribed circle, we have found out the answer.

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