GK_Gmat wrote:

Two machines are sawing wood but due to space limitations only one machine can work at a time. Machine A is used for the first shift and the other machine for the second shift, while both work for half 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 the current work schedule?

a) 14

b) 13

c) 11

d) 9

e) 7

Pls. explain your answer.

A's rate = 1/12 in 2 shifts

B's rate = 1/15 in 2 shifts

A's rate = 1/24 in 1 shift

B's rate = 1/30 in 1 shift

A's rate = 1/18 in 1.5 shifts

B' rate = 1/22.5 in 1.5 shifts

1/18 = 5/90, 1/22.5 = 4/90

9/90(t)=w, t=10

b/w C and D