GMAT Prep: Work-Rate

[chetanojha]

Please help me to understand this.I couldn't answer this question in the prep test. Can somebody tell me what/how much impact it would have make on my score. I managed to score only 520 in the test and this is the only one question which I didn't answer before time out.

[humans]

Machine A rate = 9000
Machine B rate = 7000

Total pencils required = 100,000

Least amount of B will be when machine A work for full 8 hour and remaining will be completed by machine B

So A will make 72000 pencils in 8 hours
Now will need to make remaining 28000 pencils which it can make in 28000/7000 = 4 hours