Hope this is understandable.
18 - 12 = 6. Because the top legs of the 2 triangles are congruent, you divide 6 / 2 to get 3 as their length. Then you can use the Pythagorean Theorem to solve for x.
3^2 + x^2 = 14^2
9 + x^2 = 196
x^2 = 187
x = sqrt(187)
Its 12 because you do 10.5/21=x/24
90+80+90+x=360
260+x=360
x=100
A suitable probability calculator will tell you that probabilty is about 25.9%.
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The normalcdf function on this calculator gives an area between a lower and upper limit. If the upper limit is more than about 8 standard deviations above the mean, the 10th decimal place is unaffected. Since the standard deviation here is about 5, we need to have an upper limit that is about 40 above the mean of about 30. We have chosen 80 as a nice round number and to give a little more margin.