Question 232532
Let n = no. of nickels
Let d = no. of dimes
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Write an equation for each statement:
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"Winthrop has $4.40 worth of nickels and dimes."
.05n + .10d = 4.40
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" If he has 3.5 times as many nickels as he does dimes,"
n = 3.5d
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 how many dimes does he have?
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If we substitute 3.5d for n in the 1st equation we only have one unknown
That would make it easy.
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.05(3.5d) + .10d = 4.40
multiply whats inside the brackets
.175d + .10d = 4.40
.275d = 4.40
divide both sides by .275
d = {{{4.40/.275}}}
d = 16 dimes
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Remember the 2nd statement: "he has 3.5 times as many nickels as he does dimes"
n = 3.5(16)
n = 56 nickels
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Check this 
.05(56) + .10(16) =
2.80 +  1.60 = 4.40, confirms our solutions
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Did this make sense to you?