Problem 10
Answer: RS = 17.5
This is because the opposite sides of any parallelogram are congruent (PQ = RS and QR = PS)
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Problem 11
Answer: QT = 18
Diagonal QS is cut in half (bisected) by the other diagonal PR. The smaller pieces QT and ST are the same length.
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Problem 12
Answer: angle PQR = 70 degrees
The angle QRS is given to be 110 degrees. Angle PQR adds to this to get 180, so we effectively solve x+110 = 180 to get x = 70. Any pair of adjacent angles of any parallelogram are supplementary.
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Problem 13
Answer: 110 degrees
Opposite angles of a parallelogram are congruent. Angle SPQ is opposite of angle QRS = 110.