Hi Nancy;
FIRST EQUATION...P+C+D=30
SECOND EQUATION...P+C=D+6
4P+2C+2D=80
Let's divide both sides by 2...
THIRD EQUATION...2P+C+D=40
Let's begin with the second equation..
P+C=D+6
This is also...
P+CD=6
Let's add the first equation and second equation....
P+CD=6
+(P+C+D=30)
2P+2C=36
Let's divide both sides by 2...
P+C=18we will use this again below...
The first equation was...
P+C+D=30
Let's substitute P+C with 18...
18+D=30
Let's subtract 18 from both sides...
D=12
Let's return to the third equation...
2P+C+D=40
2P+C+12=40
Let's subtract 12 from both sides...
2P+C=28
Let's subtract formulas...
2P+C=28
(P+C=18)from above
P=10
Let's return to the first equation...
P+C+D=30
10+C+12=30
C+22=30
C=8
Let's double check this with the third equation...
2P+C+D=40
2(10)+8+12=40
20+8+12=40
40=40
12/1/2013

Vivian L.
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