Question 1033682
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According to rules of posting on this web site, you may ask one question per post. I chose to answer your second question:
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2) You have 15 coins in your pocket that are either quarters or nickels. They total $2.75. How many of each coin do you have? 
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Q=number of quarters; N=number of nickels
Q+N=15
Q=15-N
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$0.25Q+$0.05N=$2.75
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$0.25(15-N)+$0.05D=$2.75
$3.75-$0.25N+$0.05N=$2.75
-$0.20N=-$1.00
N=5
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ANSWER 1: There are 5 nickels.
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Q=15-N=15-5=10
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ANSWER 2: There are 10 quarters.
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CHECK:
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$0.25Q+$0.05N=$2.75
$0.25(10)+$0.05(5)=$2.75
$2.50+$0.25=$2.75
$2.75=$2.75