Bunuel wrote:

Two machines are sawing wood but due to space limitations only one at a time can operate. Machine A is used for the first shift and the other machine for the second shift, while both work half of the third shift. Machine A can do the job in 12 days if it worked two shifts and Machine B can do the job in 15 days if it worked two shifts. How many days will it take to do the job with current work schedule?

A. 14

B. 13

C. 11

D. 9

E. 7

given rate of A = 1/12 for two days so per day = 1/24 ; half day 1/48

and rate of B = 1/15 for two days , so per day = 1/30; half day 1/60

(1/24 + 1/30) + (1/48 + 1/60) = 80/9 days =

9

IMO D

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