Find the average of the unknown angles

In the triangle shown below, what is the average of angles a,b,c,d, and e?

A.)  30°
B.)  45°
C.)  54°
D.)  60°
​E.)  72°

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The angles of a triangle add up to 180°.
Therefore, the angles of two triangles add up to 360°.

The only known angle is 90°.
The unknown angles add up to 270°.   {subtracted the known angle, 90°, from the sum of all the angles, 360°}

a + b + c + d + e = 270

a + b + c + d + e
----------------  =  average of the five angles   {added numbers and divided by the number of numbers}
            5

270
-----  =  average of the five angles   {substituted 270 for the sum of the unknown angles}
  5

54 = average   {divided}

C.)  54°

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