y = number of yellow marbles
r = number of red marbles
The total number of marbles
y+r=61
The number of yellow marbles compared to red.
y=2r+4
Substitute the second equation into the first equation because they are the same variable and, therefore, are the same number.
2r+4+r=61
Simplify
3r+4=61
Subtract 4 from both sides
3r=57
Divide both sides by three
r=19
We have the number of red marbles, but we are looking for the number of yellow marbles, so plug the 19 red marbles into either equation.
y=2(19)+4
or
y+19=61
Simplify
y=42
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Answer:
42 yellow marbles
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Equations:
y+r=61
y=2r+4
