Answer:- m∠S=78°.
Explanation:-
Given:- Two congruent triangles ΔMNP and ΔQST which are congruent, where m∠M=66° and m∠N=78°.
⇒The corresponding parts of ΔMNP are congruent to the ΔQST .
Therefore, all the corresponding sides of ΔMNP are congruent to the ΔQST and all the corresponding angles of ΔMNP are congruent to the ΔQST such that
MN=QS , NP=ST and MP=QT.
m∠M=m∠Q, m∠P= m∠T and m∠N=m∠S.
Substitute the value of m∠N=78°, we get
m∠S=78°.
