Thank you for the opportunity to help you with your question!
If we remember that the order of the letters in the triangle name tell us which parts of each triangle are congruent, then we can see that <A is congruent to <D because they are in the same order in the letters in the name of the triangle. ( ^ABC = ^ DEC. A and D are in the same spot in the order of letters). So, if <A = 40 degrees, the m<D is 40 degrees.
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