The nation's 17-year-old students are dismal at math, with only half able to cope with problems usually taught in junior high school, a testing agency reported Tuesday.

The federally sponsored National Assessment of Educational Prog-ress also said that nearly 27 percent of 13-year-olds are lacking in basic computational skills.More than a quarter of the 17-year-olds reported that they did not usually understand what was talked about in mathematics class, and only 6 percent could solve problems that took several steps or involved algebra or geometry.

The agency reported the results of tests it gave nearly 15,000 students ages 9, 13 and 17 in 1985-86, as well as comparative data from math assessments in 1973, 1978 and 1982.

The report, "The Mathematics Report Card: Are We Measuring Up," said all three age groups improved recently, but mostly on low-level skills and the 17-year-olds still have not regained all the ground they lost in the 1970s.

The report said 98 percent of the 9-year-olds and 100 percent of the older students knew simple arithmetic facts, such as 39-26=13.

But virtually none of the younger students and only 6.4 percent of the oldest could solve multistep problems and algebra questions.