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Number of hours she works=2¹/₂=5/2
Number of days=7
.·.Number of hours consumed=5/2×7=35/2=17.5 hours 
If we round off the answer is 17 hours.
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Rehana works 2 1/2 hrs = 150 minutes
As she completed her work in 7 days,
150×7=1050 minutes =17.5 hours
Ans : Rehana took 17.5 hours to complete her work.
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