Question 88429
There are several ways you can do this problem. Here's one way.
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You are told that x = 3s and also that x = a + 15. Therefore, you can tell that since
3s equals x and a + 15 equals x, then 3s must also equal a + 15. In equation form this is:
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{{{3s = a + 15}}}
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Solve this equation for s by dividing both sides by 3 to get:
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{{{s = (a + 15)/3}}}
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Then you are also told that s = a - 1. So you can substitute a - 1 for s in the equation to get:
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{{{a - 1 = (a + 15)/3}}}
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Get rid of the denominator by multiplying all terms on both sides by 3 to get:
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{{{3a - 3 = a + 15}}}
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Subtract a from both sides and the equation becomes:
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{{{2a - 3 = 15}}}
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Add 3 to both sides:
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{{{2a = 18}}}
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Solve for a by dividing both sides by 2:
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{{{a = 9}}}
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So now you know the value of a.  And the problem originally told you that:
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{{{x = a + 15}}}
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Substitute 9 for a and you have:
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{{{x = 9 + 15 = 24}}}
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So the answer to your problem is that x = 24.  You also know that a = 9, and since 
s = a - 1, you know that s = 9 - 1 = 8.  And since you were told that x = 3s you can check
by substituting 8 for s and get x = 3s = 3*8 = 24.  So that checks the problem also.
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Everything seems to work which helps to confirm that x = 24 is correct.