Convert the information about each laborer to a rate. The first laborer, stuffing 1,000 envelopes in 2 hours, stuffs 500 envelopes per hour. The second, stuffing 1,000 envelopes in 2.5 hours, stuffs 1,000/2.5 = 400 envelopes per hour. Combined, then, they stuff 900 envelopes per hour.

If they can stuff 900 envelopes per hour, that means it takes 1/3 as long (1/3 of an hour) to stuff 1/3 as many envelopes (300 envelopes). 1/3 of an hour is 20 minutes, choice (B).