Question 359419
average = {{{sum(x)/n}}}  therefore 
{{{n*average=sum(x)}}}
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given
average=120 then we have the relationship  {{{n*120=sum(x)}}}
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also if one number is increased by 300 (this means that {{{sum(x)}}} increases by 300) and the average =135
thus   {{{n*135=sum(x)+300}}}
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So you have two equations and two unknowns, solve for n

{{{n*120=sum(x)}}}
{{{n*135=sum(x)+300}}}
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{{{n*135-n*120=sum(x)+300-sum(x)}}}  subtract the second equation from the first
{{{15n=300}}}
n=300/15=20