Average daily wage of a worker for 15 consecutive working days = 90$

During the first 7 days , the daily average daily wage = 87$

During the last 7 days , the daily average daily wage = 92$

Wage on 8th day = 90*15 -( 87*7 + 92 *7)

= 1350 - (609 + 644)

=1350 - 1253

= 97

Answer D

Alternatively ,

During the first 7 days , since the daily average daily wage = 87$

Amount by which sum of first 7 days varies from average for entire 15 days = -3*7 = -21

During the last 7 days , since the daily average daily wage = 92$

Amount by which sum of last 7 days varies from average for entire 15 days = 2*7 = 14

Amount by which sums first 7 and last 7 days varies from average for entire 15 days = -21 +14 = -7

Therefore , in order to make average for entire 15 days to be 90 , the wage on 8th day needs to be = 90 + 7 = 97

Answer D

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