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kaylaholman15
The answer would be 42° because a triangle equals 180° and you know 2 angle measurements, 48 and 90.
48 + 90 = 138
180 - 138 = 42°
(You know one angle is 90° because supplementary angles equal 180°)

We use two angle sums:

---The sum of the angles of a straight line is 180 degrees.

---The sum of the angles inside a triangle is 180 degrees


There are two triangles present. The smallest (left) triangle has a right angle at the top (straight line angle sum). So in that triangle, we know that y + 48 + 90 = 180.  Now we move to algebra class.

y + 48 + 90 = 180.

y + 138 = 180            add common terms on the left

y = 42                     subtract 138 on both sides


Thus, y is 42 degrees.  

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