The least number of cases and envelopes so that there would be no remainder can be determined by finding their least common multiple. The solution is as follows
4 | 20 12
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| 5 3
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Multiplying 4*5*3 = 60. The least common multiple is 60. Thus, there should be at least 60 each of the envelopes and the packets.
Number of mailing envelopes = 20x = 60
x = 3
Number of packets = 12y = 60
y = 5
That would be 3 packs of the envelopes, and 5 packs of the packets.
