I took a longer approach, but an easier one-
A took 3hours(hrs) to process 6000 envelopes(e) --> equation 1, eq1
B+C took 2.5hrs to process the same no. of e i.e. 6000 --> eq2
A+C took 1hour(hr) to process 3000 e --> eq3
B took ?hr to process 12000 e --> this is the ask
From eq1,
A took, 3hr = 6000 e
so, in 1hr = ? e
Cross multiplying, A processed 2000 e in 1hr --> eq4
Substituting eq4 in eq3,
A+C took 1hr = 3000 e
A = 2000 e
so, C = 1000 e, in 1hr --> eq5
From eq2,
B+C took 2.5hr = 6000 e
so in 1hr = ? e
Cross multiplying, B+C processed 2400 e in 1hr --> eq6
From eq5,
C processed 1000 e in 1hr
Hence in eq6,
B+1000 = 2400
or B processed 1400 e in 1hr --> eq7
The ask is how many hours B took to process 12000 e
From eq7,
B took 1hr to process 1400 e.
1hr = 1400 e
?hr = 12000 e
Cross multiplying, it would take 12000/1400 = 60/7hrs to process 12000 e