Here in Financial Central, donations to Children's Hospital take on a familiar pattern. Checks tend to be made out for round numbers -- here a 10, there a 20, occasionally a 50 or a 100. It's a rare check that contains dimes or pennies, and a rarer one whose dollar amount isn't divisible by five.

That's why a check for $47.88 from Takoma Park made me stop the other day.

"Hmmmmm," I said, deliberately setting aside the explanatory letter so I wouldn't peek at it. "Let's limber up the old gray matter and see if I can guess.

"It's from some guy who hit the daily double at Bowie and had that much left after two hot dogs and a beer.

"Or it's a very precise lady who can't stand it if her checking account balance isn't rounded off. So she's sending the 'leftovers.'

"Or maybe it's a precocious eight-year-old who bought one share of Fiddlefaddle Industries and doubled his money. After all, $47.88 is 47 7/8 rounded to the nearest penny."

Well, I needn't have troubled my fevered brain. The truth was, it was all a mistake, made by a young fellow who begged to remain anonymous, for reasons that will soon become obvious.

"I'm a college student who works at a large department store auto center putting tires on cars part-time," my $47.88 donor wrote.

"I noticed that my paycheck seemed bigger than it should be last week. Sure enough, it seems someone made a mistake and reported me working 5.7 hours more than I really did last week.

"Well, now, what to do? If I keep the money ($23.94) I will not feel too good about it, even though I am overworked and underpaid (at least I think so, anyway). But if I mention the mistake, they'll think I'm nuts.

"Well, I visited the personnel office to report the mistake anyway, but it was closed early that day for some reason. 'Aha!' I thought, 'This is an omen. I will give the money to Children's Hospital and match it with an equal amount of my own money.'"

You may be overworked and underpaid, Mr. Tire Putter Onner. But you sure aren't undergenerous.

Last year, Wallace Erwin of Northwest laid down a challenge. He said he would double his donation to Children's Hospital if, during 1982, he counted fewer than 100 instances in which the word "whom" was used incorrectly in the pages of The Post.

Well, it seems that those of us who (not whom!) toil here at Wordland have learned our lessons. "The Post's bad 'whoms' have decreased markedly in the past year, totalling far below 100 by my count," Wallace writes. Therefore, true to his word, he sent along a check for $50, double last year's $25.

Next year? "If I can just get The Post to stop writing 'any more' as one word . . . ," Wallace says.

Actually, the dictionary calls this one a tossup, Wallace, and The Post's own stylebook is silent on the question. Like you, I prefer "any more." I wouldn't write "allthetime" or "somewhereelse," so I see no reason to make "any more" a Siamese twin just because "any" and "more" form one thought as soon as you put them back to back.

But grammar for its own sake is one thing and grammar for the benefit of Children's is another. I'll fight the good fight from my perch inside these walls, Wallace. If I'm successful, and you send $100 next year, I won't bother you anymore (you knew I'd do it!).