Question 140436
A jar has twice as many red marbles as white marbles
Eq: r  -2w + 0 = 0  
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and the ratio of blue marbles to white marbles in the jar is 3:5
Eq: 0 - 5w + 3b = 0
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There are only red, white and blue marbles in the jar.
If there are 2100 more red marbles than blue ones
Eq: r + 0 -b = 2100
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how many marbles are in the jar altogether? 
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You have three equations with three unknows:
Solving the set of equations you would get:
r = 12600 red marbles
w = 6300 white marbles
b = 10500 blue marbles
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Cheers,
Stan H.