Calculate the value of the exterior angle

What we know is that the sum of a triangle's internal angles are 180º, and a straight angle makes 180º.

first, we have to find the third internal angle(let's call it n) so that we can use it to find the exterior angle.

(3x + 20) + (4x + 5) + n = 180

simplify:

7x + 25 + n = 180

7x + n = 155

n = 155 - 7x

Now we can add it to the external angle to find x.

(155 - 7x) + (8x + 15) = 180

simplify:

170 + 8x - 7x = 180

170 + x = 180

x = 10

Now we can substitute it to the external angle.

8 x 10 + 15 = 95

the exterior angle is 95º.

step-by-step explanation:

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