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let G be number of days in which Dr. gupta can complete the evaluation of answer sheet , let Sh be number of days in which Dr. Sharma
can complete the evaluation of answer sheet , let Si be number of days in which Dr. Singh can complete the evaluation of answer sheet
Given Dr. Gupta is 40% more efficient than Dr. Sharma therefore number of days required by Dr. Sharma to complete the evaluation of
answer sheet = 1.4*G
Dr. Gupta takes 10 days less than Dr. Sharma to complete the evaluation work i.e. 1.4G-G = 10 therefore G = 25 and hence Sh = 35
Also given that Dr. Sharma is 20% more efficient than Dr. Singh therefore number of days required by Dr. Singh to complete the evaluation
of answer sheet = = 42 i.e. Si = 42
Dr. Gupta starts the evaluation work and works for 10 days therefore he completes of work then Dr. Sharma takes over. Dr. Sharma
evaluates for next 15 days therefore he completes of work. Fraction of work left is 1 - 10/25 - 15/35 = 6/35 now time required by Dr. Singh to
complete this amount of work is = 7.2 days
Therefore our answer is option 'A'