As Paul Slepian {letters, Feb. 15} indicates, the mathematical competency of our high school graduates has been deteriorating. So has that of our college graduates. Prof. Slepian may well be correct in laying some of the blame on standardized tests; the best that can be said for teaching testing techniques is that the students learn something.

The basic problem, I believe, is the pitiful low to which academic standards have fallen, especially in mathematics. Unfortunately, many ''successful'' algebra students can neither recognize nor understand the addition of polynomials used to illustrate Prof. Slepian's letter. Many students ''pass'' all the required courses and graduate without learning very much. They are given the credits, diplomas and degrees, but not the knowledge they need for success.

There are few teachers at any level who can afford to give failing grades to all the students who do not deserve to pass. Yet some students who have passed high school algebra courses do not know what an equation is, let alone how to solve it. After more algebra at the college level, some do no better.

With the realization that grade inflation is not helping the student, lip service is paid to maintaining academic standards. However, administrative pressure to maintain an acceptable ''success rate'' means that anything goes. Some teachers grade too easily or on a ''curve,'' lower the grade boundaries, do not cover the prescribed curriculum or use other insidious techniques of grade inflation.

But the fault does not lie entirely with the curriculum, the faculty or the administration. In many cases, especially at the college level, students must earn a living as well as attend school. Many students fail simply because they do not have the time to devote to their studies. The only realistic choice for such people is to spread their education over more years, earning the meaningful grades that come with higher standards.

The solution to this problem requires establishing and measuring standards, which is not easy. Community colleges and other institutions with open admission not requiring standardized tests must bear the brunt of the remedial work required. Open admission is fine, providing it does not imply open graduation. Meaningless credits, diplomas and degrees mean that more and more of our young people are drowning in the ''rising tide of mediocrity.'' DAVID A. SWICK Accokeek, Md.