Question 188885
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Let <font color=blue>x</font> = 13 cent stamps
And <font color=blue>y</font> = 20 cent stamps


Then, <font color=blue>x + y =60 </font>, Eqn 1


Also, <font color=blue>0.13x + 0.20y = $10.25</font>, Eqn 2


In Eqn 1 you get, <font color=blue>y = 60 - x</font>, Eqn 3.
Substitute Eqn 3 in Eqn 2:
{{{0.13x+0.20(highlight(60-x))=10.25}}}
{{{0.13x+12-0.20x=10.25}}}
{{{12-10.25 = 0.20x-0.13x}}}
{{{1.75=0.07x}}} -----> {{{cross(1.75)25/cross(0.07)=cross(0.07)x/cross(0.07)}}}
{{{highlight(x=25)}}}, Number of 13cent stamps ( Answer )


Go back Eqn 3:
{{{y=60-25=red(35)}}}, Number of 20 cent stamps


Let's check, go back Eqn 2:
{{{0.13(25) + 0.20(35) =10.25}}}
{{{3.25+7.00=10.25}}}
{{{10.25=10.25}}}


Thank you,
Jojo</font>