Triangle ABC is congruent to triangle DEF. The order of the lettering is important because it tells us how the angles pair up. We see that angle B and angle E are the second letters of ABC and DEF respectively. Because the triangles are congruent, this leads to angle B = angle E.
Angle B is shown to be 20 degrees, so,
angle E = angle B
2(m-15) = 20
m-15 = 20/2
m-15 = 10
m = 10+15
m = 25
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A slightly alternative way to solve:
2(m-15) = 20
2m-30 = 20
2m = 20+30
2m = 50
m = 50/2
m = 25
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Checking the answer:
2(m-15) = 20
2(25-15) = 20
2(10) = 20
20 = 20
We get the same thing on both sides, so we end up with a true equation. This means the first equation is true when m = 25. The solution has been confirmed.
