SURELY EVERYONE agrees that a 13-year-old child -- a seventh grader -- ought to be able to tell you what two-thirds of 9 equals. But, in fact, it appears that fewer than half of all 13-year-olds can answer that one. This fragment comes from the National Assessment of Educational Progress, a federally supported research organization that gives tests and reports the results -- generally depressing -- to parents and taxpayers. Over the past five years, the National Assessment has found, there has been a perceptible drop in the ability of American schoolchildren to do simple arithmetic.

That conclusion is, unfortunately, altogether consistent with the long decline in the scores that high school students have been getting on the College Board's Scholastic Aptitude Test. After much analysis, a College Board committee concluded two years ago that the decline had a lot to do with the peculiar cultural currents of the late 1960s and early 1970s.Since the end of the Vietnam War, the atmosphere in the schools and colleges has become a great deal more businesslike -- but the SAT scores have dropped again this year. It was a very small drop, but it was visible. Could the turbulence of the 1960s have any effect on the progress in arithmetic of 13-year-olds today? None directly -- but, of course, some of the college students of that period are now junior high school teachers.

The poor results of the math test are also consistent with the grades in the basic competence tests now being introduced as a requirement for high school graduation in many states, including Virginia and Maryland. Competence testing makes it clear that these children's difficulties are not limited to formal textbook drills, but extend to the everyday exercises of the paycheck and the light bill.

Some specialists speculate that the deterioration is related to the back-to-basics movement in school curricula. That suggestion seems utterly implausible. The return to basics has been recent, and not sufficiently widespread, to account for a decline as general as this one.

It's an oddity: American prosperity increasingly depends on advanced technology. Business increasingly depends on highly sophisticated accounting systems. Everybody's personal taxes and household finances are getting more complex -- not to mention the indescribable Metro fare formulas. And through it all, children's ability to deal with numbers is declining.

The only bright sign is the country's rising interest in this kind of testing, as a measure of what goes on in the classroom. The very existence of the National Assessment is a welcome indication that people are now prepared to hold their schools to a tighter measure of performance. These tests constitute a legitimate report card for the country's system of education. In math this year, the grade is somewhere between a C-minus and a D-plus.