Bunuel wrote:

In the equilateral triangle RST above, y equals

(A) 60

(B) 70

(C) 75

(D) 80

(E) 85

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Sum of angles1. x = 20

Because ∆ RST is equilateral, ∠ R, ∠ S, and ∠ T = 60

∠ S = 60 = 3x

x = 20

2. ∠ R = 60, from above

3. For leftmost small triangle, sum of angles equals 180:

x + ∠ R + third angle = 180

20 + 60 + third angle = 180

Third angle = 100

Call third angle z

4. y equals?

y and z are on a straight line

y + z = 180

y + 100 = 180

y = 80

Exterior angle1. y = exterior angle

For small triangle on the far left with one side RS, exterior angle y = sum of interior angles x + ∠ R

2. Angles R and x

Because ∆ RST is equilateral, ∠ R, ∠ S, and ∠ T = 60

∠ R = 60∠ S = 60 = 3x, so

x = 203. y equals?

y = x + ∠ R

y = 20 + 60

y = 80

Answer D
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