If m∠A=m∠B and m∠A+m∠C=m∠D, then m∠B+m∠C=m∠D. Which property is shown?

Since m∠A = m∠B, we can replace m∠A with m∠B in the equation m∠A + m∠C = m∠D. This gives m∠B + m∠C = m∠D, which shows the substitution property. The correct choice is substitution property.

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