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2014-05-06T00:08:33+03:00
(2a-20):30+13=x
75:x=5 rest 0
75=5x+0
75=5x
x=15

(2a-20):30+13=15
(2a-20):30=15-13
(2a-20):30=2
2a-20=2*30
2a-20=60
2a=60+20
2a=80
a=40