Machine A can process 6000 envelopes in 3 hours. Machines B and C working together but independently can process the same number of envelopes in 2.5 hours. If Machines A and C working together but independently process 3000 envelopes in 1 hour, then how many hours would it take Machine B to process 12000 envelopes. Choose the most appropriate option
A) 2  B) 3  C) 4  D) 6  E) 60 / 7

Rationale: for 1 hour - machine a rate - 2000 envelopes machine b + c rate - 2400 envelopes since a + c = 3000 envelopes a ' s rate is 2000 envelopes as above , c has a rate of 1000 envelopes per hour . which makes machine b ' s rate as 1400 envelopes per hour . thus , it will take 8 hours to manufacture 12000 envelopes . answer : - e
Answer: E