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Let the amount he invested at 11%=x
Let the amount he invested at 13%=y
He invested the same amount in both stocks, so x=y
Interest(i)=principle(p)*rate(r)*time(t), but unless they specify that the time is different than one, i=p*r.
So the amount of interest he gets at 11%=.11x
The amount of interest he gets at 13%=.13y
Add the two together and your interest is=840
Therefore your system of linear equations is:
----------------Substitute the first equation into the second and you have:
x=$3,500 (the amount he invested at 11%)
y=x so y=$3,500 (the amount he invested at 13%)
Add the two investments together to find out how much he invested all together:
$3,500+$3,500=$7,000 is the amount he has invested altogether.