Task: Question Answering
Input (English): Problem: 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 2800 envelopes . 
Options: a ) 2 , b ) 3 , c ) 4 , d ) 6 , e ) 60 / 7
Output (English): a