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a in (-oo:+oo)
a/4.2 = 5.9+13.5 // - 5.9+13.5
a/4.2-13.5-5.9 = 0
0.23809524*a-19.4 = 0 // + 19.4
0.23809524*a = 19.4 // : 0.23809524
a = 19.4/0.23809524
a = 81.47999935
a = 81.47999935
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