Machine A can stuff a batch of envelopes in 10 hours.
Machine B can stuff a batch of envelopes in 5 hours.
So in 1 hour Machine A and B can complete 1/10 and 1/5 of total envelopes respectively.
So together they can complete 1/10+1/5 or 3/10th of batch of envelopes.
So it will take them 10/3 hours to complete a batch of envelopes.