Ruth Yap Hoy, 79, a retired mathematics professor at Marymount College in Arlington, died of pneumonia Monday at George Washington University Hospital.

She and her husband of 42 years, Elvin A. Hoy, a retired mathematician for the Department of the Army, lived in Arlington for more than 30 years. He died on Feb. 16.

A native of Honolulu, Mrs. Hoy earned a bachelor's and master's degrees in mathematics from the Univeristy of Hawaii and then taught mathematics there for about 15 years before World War II. She and her husband met at the university, where he taught mathematics and engineering drawing.

In the early 1940s, the Hoys left Hawaii to attend Columbia University in New York. They moved to Washington when Mr. Hoy went to work for the War Production Board during World War II.

Mrs. Hoy headed the mathematics department at Wakefield High School in Arlington for a number of years before joining the faculty at Marymount in 1967.She retired in 1972.

She was a life member of the Hawaii State Society, a member of the Arlington Committee of 100, a former treasurer of the Women's Committee of the Arlington Symphony Association and a member of the Aurora Hills Women's Club in Arlington.

The Hoys had two sons, Charles W., who died in 1964, and Army Capt. Robert E., who died in Vietnam in 1968.

Mrs. Hoy's survivors include two sisters, Rachel Yap of Boca Raton, Fla., and Hannah Loving of Honolulu, and three brothers, Charles Eugene and Gerald Yap, all of Honolulu.

The family suggests that expressions of sympathy be in the form of contributions to the Potomac Chapter of the Myasthenia Gravis Foundation Inc., or to the American Cancer Society.