SOLUTION: I have fifteen coins. Some are valued at 5 cents and others at 20 cents. Their total value is $1.65. How many 5-cents coins and 20-cents coins do I have?

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Question 570131: I have fifteen coins. Some are valued at 5 cents and others at 20 cents. Their total value is $1.65. How many 5-cents coins and 20-cents coins do I have?
Answer by Maths68(1474) (Show Source):
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Total value of the all coins = $1.65
Total number of coins = 15
Value of a 5 cents coin = 5/100 = $0.05
Value of a 20 cents coin = 20/100 = $0.2

Let
You have total number of 5 cents coins = x
You have total number of 20 cents coins = 15-x

Solution
(Value of a 5 cents coin)(number of 5 cents coins)(Value of a 20 cents coin)(number of 5 cents coins)=Total value of the all coins
(0.05)(x)+(0.2)(15-x)=1.65
0.05x+3-0.2x=1.65
0.05x-0.2x=1.65-3
-0.15x=-1.35
-0.15x/-0.15=-1.35/-0.15
x=9

You have total number of 5 cents coins = x = 9
You have total number of 20 cents coins = 15-x = 15-9 = 6

Check
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Value of 5 cents coins = (0.05)(9)=0.45
Value of 20 cents coins = (0.2)(6)=1.2
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Sum = 1.65
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