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2014-04-28T17:16:12+03:00
[(2x-20):30+13]:75=5
(2x-20):30+13=5*75
(2x-20):30+13=375 
(2x-20):30=375-13
(2x-20):30=362
2x-20=362*30
2x-20=10860
2x=10860=20
2x=10880
x=10880:2
x=5440