Question 646190
Suppose that Maria has 150 coins consisting of pennies, nickels, and dimes. The number of nickels she has is 9 less than twice the number of pennies; the number of dimes she has is 21 less than three times the number of pennies. How many coins of each kind does she have? 
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Equations:
p  +  n  +  d  = 150
n = 2p  - 9
d = 3p - 21
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Substitute for "n" and for "d" and solve for "p":
p + 2p-9 + 3p-21 = 150
6p = 180
p = 30 (# of pennies)
n = 2p-9 = 51 (# of nickels)
d = 3p-21 = 69 (# of dimes)
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Cheers,
Stan H.
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