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[tex]\\ \sf\longmapsto m=\dfrac{f}{f-u}[/tex]
[tex]\\ \sf\longmapsto 3=\dfrac{20}{20-u}[/tex]
[tex]\\ \sf\longmapsto 3(20-u)=20[/tex]
[tex]\\ \sf\longmapsto 60-3u=20[/tex]
[tex]\\ \sf\longmapsto 3u=40[/tex]
[tex]\\ \sf\longmapsto u=13.3cm[/tex]