Join AC and PQ.
Since AC and PQ are diagonals of parallelogram ABCD and parallelogram BPQR respectively.
Therefore, Area (ABC)=12Area(ABCD) ...... (1)
and Area (PBQ)=12 Area(BPRQ) ... (2)
Triangles ACQ and AQP are on the same base AQ and between the same parallels AQ and CP.
ar(ACQ)=ar(AQP)
⇒ar(ACQ)–ar(ABQ)=ar(AQP)–ar(ABQ) (Subtracting ar(ABQ) from both sides)
⇒ar(ABC)=ar(BPQ)
12ar(ABCD)=12ar(BPRQ) (Using (1) and (2))
⇒ar(ABCD)=ar(BPRQ)