Question 181305
Set up a moment (force x distance) equilibrium equation. 
{{{F[1]d[1]=F[2]d[2]}}}
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Find the associated distances from the pivot for both forces. 
{{{d[1]=2.5}}}
{{{d[1]+d[2]=3}}}
{{{d[2]=0.5}}}
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The force, in this case, is gravity,
{{{F=mg}}}
{{{m[1]gd[1]=m[2]gd[2]}}}
{{{m[1]d[1]=m[2]d[2]}}}
{{{90(2.5)=m[2](0.5)}}}
{{{m[2]=90(2.5/0.5)}}}
{{{m[2]=90*5=450}}}
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B.)450 kg.