Well, this is how I would solve the problem:
Sally takes 4k+7 hrs to complete the thing alone.
So time taken by Sally = 11, 15, 19, 23 or 27 hrs.
We know nothing about how fast George and Andy work!
Thus - NOT SUFFICIENT!
George - 2k+1 hr
Time - 3, 5, 7, 9 or 11
Andy - 3+2k
Time - 5, 7, 9, 11 or 13
Assume the worst case scenario - Only George and Andy are working and they're both taking their own sweet time:
Therefore, time : 13x11/(13+11) = 13x11/24 = 143/24
24x8 = 192
Thus, 143/24 is less than 8 :D
George and Andy can finish the job before the 8hr deadline if they work together (irrespective of how fast or slow they work and independent of Sally's work rate!)
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From 650 to 710 to 750 - My Tryst With GMAT [Experience Thread]