Two in every five.
In the 1920s, the percentage of women in the USSR's workforce was about 1 in 4 -- remaining around 28% through most of that decade. But those numbers would increase under the First and Second Five-Year Plans instituted by Joseph Stalin. By 1935, the percentage of women employed in large-scale industry in the Soviet Union had grown to 40% (representing two out of every five workers).
For further information and detail, you might consult the book, The Position of Women in the USSR, by G.N. Serebrennikov (published 1937).
