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a in (-oo:+oo)
1/4 = a/20 // - a/20
1/4-(a/20) = 0
1/4-1/20*a = 0 // - 1/4
-1/20*a = -1/4 // : -1/20
a = -1/4/(-1/20)
a = 5
a = 5
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