The Stalinist command economy, in which market forces were suppressed and all orders for production and distribution came from the state authorities, survived in all its essential features until the fall of the Soviet regime in 1991. In the early stages of the communist economic revolution, Armenia underwent a fundamental transformation into a "proletarian" society. Between 1929 and 1939, the percentage of Armenias work force categorised as industrial workers grew from 13% to 31%. By 1935 industry supplied 62% of Armenias economic production. Highly integrated and sheltered within artificial barter economy of the Soviet system from the 1930s until the end of the communist era, the Armenian economy showed few signs of self-sufficiency at any time during that period. In 1988 Armenia produced only 0.9% of the net material product of the Soviet Union (1.2% of industry, 0.7% of agriculture). The republic retained 1.4% of total state budget revenue, delivered 63.7% of its NMP to other republics, and exported only 1.4% of what it produced to markets outside the Soviet Union.
Question: How many percentage points did the Armenian work force categorized as industrial workers grow from 1929 to 1939?
Answer: 18