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Susan has some $2-coins and $5-coins. If there are 18 coins and the total amount of these coins is not less than $75, find the minimum number of $5-coins.

  • Math -

    Let x = 2$ coins
    Let y = 5$ coins

    First equation is for the total amount of coins:
    x + y = 18

    Second Equation is for total amount in dollars in total from the coins:
    2x + 5y = $75

    Lets isolate for y
    x = 18 - y

    Sub into second equation
    2(18 - y) + 5y = 75
    36 - 2y + 5y = 75
    3y = 39
    y = 13

    Therefore, you must have at least 13 $5 coins.

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