How to find the perimeter of a 30 60 90 Triangle? 
A $24$
B $25$
C $28 $
D $30$
E $36$
So far I have tried $A+B+C$ to find the perimeter. However my efforts were in vain for I keep on getting 28 as my answer. I have tried $6\sqrt 3 + 6 + 12=28$ Although the answer is $30$
 A: Do you see the equilateral triangle in the figure?
A: The triangle cut from the figure is equilateral, because it's interior angles must all be $60^\circ$.  Why? because $90^\circ - 30^\circ = 60^\circ$ for each of the adjecent lower angles. The sum of the measures of a triangle is $180^\circ$, so the remaining "top" angle must be $180 - 60 - 60 = 60^\circ$, too.
The side of the equilateral triangle (base) is known to be $6$, because the given figure is a square (missing bottom side) with side lengths all $6$. Since it's an equilateral triangle, the other two sides are length $6$ as well. (For an equilateral triangle: side = side = side, angle = angle = angle.)
The perimeter of the equilateral triangle (if we "added" the missing base) is 3 times the length of one of its sides: $$3 \times 6 = 18.$$
The permimeter of the given figure, as drawn, is $$5 \times 6 = 30$$ since all five edges are of length $6$:

*

*three are the 6-unit edges of a $6 \times 6$ "square-with-missing-side," and


*two are the 6-unit edges of an "equilateral-triangle-with-missing-side"
So $3 \times 6 + 2 \times 6 = 5 \times 6 = 30:\;$ this matches choice $(D)$.

