You would multiply 15 ×8= 120 kids tried out
let's say that number is "x", so that is then the 100%.
if 150 is 25%, what is "x"?

Let the sides of the polygon (which is a triangle, by the way) be x, y and z. The sum of x, y and z is the perimeter of the original poly, and this equals 18 cm.
Letting f be the scale factor, f(18 cm) = 12 cm. Then f=2/3.
The dilation reduces the size of the polygon by a factor of 1/3, producing a similar polygon which is 2/3 the size of the original one.
In each case we have 3 side lengths but no angles. We can use Heron's formula to obtain the area in each case. Look up Heron's formula. In one version of this formula, p is half the actual perimeter, meaning that p is 18 cm / 2 for the first triangle and 12 cm / 2 for the second.
The area of the first triangle would be
A18 = sqrt( 9(9-x)(9-y)(9-z) )
whereas
A12 = sqrt( 6(6-x*a)(6-y*a)(6-z*a) ), where a represents the dilation factor 2/3.
Then the ratio of the areas of the 2 triangles is
sqrt( 6(6-x*a)(6-y*a)(6-z*a) )
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sqrt( 9(9-x)(9-y)(9-z) )
We know that Angle Bisector Divides an Angle into Two Equal Angles.
As UP is the Angle Bisector of Angle U, It Divides Angle U into two Equal Parts they are Angle(1) and Angle(2)
⇒ Angle(1) = Angle(2)
Given Angle(1) = 5x + 10 and Angle(2) = 3x + 14
⇒ 5x + 10 = 3x + 14
⇒ 2x = 4
⇒ x = 2
⇒ Angle(1) = 5x + 10 = 5(2) + 10 = 10 + 10 = 20
So : Measure of Angle(1) is 20