Several weeks ago, The Washington Post published a news item that said next year the United States Postal Service will switch from five-digit ZIP numbers to nine-digit numbers.

The item said, "Instead of the 1.2 million ZIP codes now in use, there will be 19.8 million."

When no correction appeared in subsequent editions, readers began to ask me what kind of new math was involved in this statement. Valerie Matthews of College Park said the number of different ZIP codes in a five-digit system would be "10 to the 5th power, or 100,000." Joseph D. Lewis of Arlington agreed that a five-digit system will produce only 100,000 combinations (don't overlook 00000). Thomas C. Teeples of Arlington also said 100,000 is the theoretical maximum and noted that there must be fewer than 100,000 in actual use because we don't use 00001, 00002, 00003 or other numbers that small. Several readers pointed out that a nine-digit ZIP system could have a billion ZIP codes, not 19.8 million.

Helen Gaiser, general manager of the USPS Consumer Response Division, says my readers are quite correct.

There are now 1.2 million listings in the ZIP Code Directory. Since all larger cities have "multiple listings" (more than one street or area served by the same ZIP number), there are many times more ZIP Directory listings than there are ZIP numbers.

The number of combinations in a five-digit system is indeed 100,000, but only about 40,000 ZIP numbers have been assigned. The lowest in use is 00601 (for Adjuntas, Puerto Rico) and the highest is 99950 (Ketchikan, Alaska). Some 60,000 unassigned numbers are being held for future use.

The four-digit "add-on" scheduled for next February will allow 10,000 additional combinations for each of the 100,000 original possibilities. We will be able to expand the potential maximum from 100,000 to 1 billion, and the 40,000 codes now in use could become 400 million.

When the nine-digit system goes into effect, USPS estimates that the present 1.2 million listings (not codes) in its directory will grow to 19.8 million listings.The greatest effect will be felt by business firms, especially those that use the mails heavily. Computer tapes will be available for them.

The average citizen will also be given appropriate help in learning and using the new system.

A few readers have already complained to me that a nine-digit number is simply more than the average mind can cope with. I used to believe that, but I don't any more. I have a 10-digit bank account number, several 10-digit telephone numbers, a nine-digit Social Security number, an eight-digit Washington Shopping Plate, a nine-digit Central Charge card, a 13-digit Visa number and a 14-digit Master Charge number. My Pepco and Washington Gas Light account numbers have so many digits I get dizzy copying them from their bills to my checks.

If you had told me 20 years ago that the human mind can cope quite well with so many digits I would not have believed you. But we are coping because we must. Computers speed our work and cut costs, and computers thrive on numbers.

To move mail faster and minimize the need for postage increases, USPS will ask for our cooperation in using ZIP codes that permit more precise routings. Inasmuch as USPS must move more than 100 billion letters a year for us, I consider the request quite reasonable. The only thing unreasonable in this matter is the suggestion that a five-digit ZIP system can produce 1.2 million combinations. THE MYSTERY DEEPENS

You may recall that a few days ago I quoted from the "Airports" subheading of the telephone book's "United States Government" listing.

The first airport listed is, "Dulles International Airport -- Eager To Make Our Environment Reclaim the Growth of Our Existence 1905 E St. SE."

It was a tossup as to which was more mystifying: the cryptic words, or the address given for Dulles Airport.

Wanda Kirkpatrick now sheds new light on the matter. She suggested, "Look at the first item on page 185 of your Washington phone book." So I turned to page 185. The first item, under D.C. Government, was, "Mental Health Administration, Mental Health Services, Emergency Mental Health Service, 1905 E St. SE."

"1905 E St. SE"? How in the world do they run a mental health service in the middle of a busy airport? Turnabout

Harold Johnson is back from a visit to his native Miami and reports that a tremendous change has taken place since he was there last. "The Cubans have taken over whole neighborhoods," he told me. "The ghetto, for instance. The first thing they did was clean it up. They work hard, pay their taxes, help each other -- and their community just grows and grows.

"They also have a sense of humor," Harold added. "On a store window on Flagler Street I saw a big sign that said, 'We Speak English Here.'"