A bachelor friend who does his own cooking and marketing used to scoff at my advice on how to economize.

When I lectured him about buying in quantity to save money, or when I urged him to read the grogery ads carefully and make a precise list of what he would buy and where the would buy it, he used to chuckle patronizingly and tell me he didn't have time for such nonsense.

"What difference do a few pennies make?" he'd say. "I buy everything in one store and save time."

"If you were a doctor or a used car salesman," I argued, "minutes saved by buying everything in one store might be turned into money. You might devote the saved minutes to curing more patients or selling more automobiles. But you have a 9-to-5 job and and all you do with your spare time is loaf or watch television. Why not invest some of that spare time in buying intelligently?"

"I don't have the patience for it," he'd say.

My friend is still a bachelor. He still does his own cooking and his own marketing. But these days he has developed the patience to read grocery ads carefully,clip cents-off coupons, send away for rebates, patronize three supermarkets regularly and make occasional detours to others for advertised specials.

On items he buys frequently, such as soap, shampoo, cotton swabs, tissues, paper towels, peanut butter, jams, jellies, oleo, milk, detergent and laundry products, he has memorized a series of unit prices that he says make marketing easier. He explains:

"When you encounter a store sign that says, 'Save! 2 for $1.77,' and there is some nice even amount like 9 1/4 ounces in each container, it's awfully handy to have a benchmark price in your head. For example, if I remember that 10 cents an ounce is a reasonable price for that product, I can do the arithmetic without a pencil and paper. Two times 9 1/4 is 18 1/2, and 18 1/2 ounces at a dime each comes to $1.85. So the special is 8 cents less than the regular price, or 4 cents off on each bottle.

"When I have that information at my fingertips, I can make a quick judgement on whether I should stock up."

Your letters are a constant reminder that my bachelor friend is not alone in his conern about rising prices. For example, when Giant Food (not Giant Foods, you WTOP newscasters, but Giant Food ) raised its Kiss Ice Cream from 99 cents a half-gallon to $1.09, and then to $1.43, a great howl went up among District Liners. There was also anguish when Giant raised Nabisco's 18-ounce Shredded Wheat from $1.25 to $1.39 and boosted its own brand of Raisin Brand from $1.09 to $1.49.

Gaint says it did its best to hold back price increases on all these items but that wholesale increases eventually reached such proportions they had to be passed along.

Sugar and butterfat are important ingredients in ice cream. Giant's Barry Scher says that sugar was $30 per hundredweight when the ice cream retailed for 99 cents. Now sugar is $52 per hundredweight, "and butterfat has experienced similar increases." Nabisco raised the wholesale price of Shredded Wheat 60 cents per case in March but Giant held firm on its retail price. In June, Nabisco boosted the wholesale price another 90 cents a case. Giant sold its remaining stock of Shredded Wheat at the old price but then had to raise the price when it restocked its shelves.

The regular price of Giant Raisin Bran was $1.39. The manufacturer had it "on special" for two weeks at $1.09, then boosted the wholesale price, so the retail price had to be raised to $1.49.

For shoppers, the story always comes out the same. Wholesalers may cut their prices temporarily, but you can bet that in the long run they will be higher.

However, steadily rising food prices are no picnic for retailers, either. As Giant's chairman of the board put it to me, "We used to be able to make a little over a penny on a dollar's worth of sales. But this year we have been absorbing so many price increases we're down to three-quarters of a penny. You'd have to spend $100 with us before we'd make 77 cents. If anybody thinks we're too quick to pass along price increases, why aren't we making more money?"

He has a point. Giant is publicly owned. Its audited sales and profit figures are made public regularly, and the bottom line speaks for itself.