What the condition says, can be expressed by this system of 2 equations x = 21y (1) x - y = 10000 tons (2) where x = sulfur dioxide amount released (in tons), and y = that of nitrogen dioxide. Substituting (1) into (2), you get 21y - y = 10000 ====> 20y = 10000 ====> y == 500. Answer. 500 tons of nitrogen dioxide were released during the study.