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Math Help - Easy Stuff?

  1. #1
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    Easy Stuff?

    I hope this is easy for you guys, because it is most certainly not for me...

    1) Brandon Filop had seven times as many pennis as Billy Wang. When Brandon gave Billy 9 pennies, he still had 36 more pennies than Billy. How many pennies did Brandon have originally?

    2)The number of digits in the number (2^{1995})(5^{1992}) is:
    a)3885
    b)1992
    c)1993
    d)1994
    e)1995

    3)The express 6\sqrt{2}+\sqrt{32} is equivalent to
    a) 7\sqrt{34}
    b) 20
    c) 10\sqrt{2}
    d) 6\sqrt{34}
    e) 6\sqrt{64}

    4) If 7^{d-4}=\frac{1}{49}, then d=
    a) 6
    b) 4
    c) 2
    d) -2
    e) -3

    5) A rectangular tank contains water to a depth of 11 cm. The base of the tank measures 5 cm by 7 cm. If all the water in the bank is poured into an empty cylindrical tank of base radius 5 cm and there is no water overflow, find the depth of water in the cylindrical tank.

    Thanks for helping me guys!
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  2. #2
    Senior Member DivideBy0's Avatar
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    1) Let F denote Brandon Filop's pennies and W denote Billy Wang's pennies.
    F=7W ...[1]
    F-9=(W+9)+36 ...[2]
    By subtraction we have
    9=7W-(W+9)-36
    9=6W-45
    6W=54
    W=9
    So F=63


    2) The number of digits of a number N is \lfloor \log N \rfloor +1

    So the number of digits in (2^{1995})(5^{1992}) is \lfloor \log{(2^{1995})5^{1992})}\rfloor+1=\lfloor \log{2^{1995}}+\log{5^{1992}}\rfloor+1=\lfloor 1995\log{2}+1992\log{5} \rfloor +1

    Then get out a calculator and that's 1993 digits.

    3) 6\sqrt{2}+\sqrt{32}=6\sqrt{2}+\sqrt{16 \cdot 2}=6\sqrt{2}+4\sqrt{2}=10\sqrt{2}

    4) 7^{d-4}=\frac{1}{49}

    7^{d-4}=\frac{1}{7^2}

    7^{d-4}=7^{-2}

    d-4=-2

    d=2

    5) The rectangular tank contains 11 \cdot 5 \cdot 7=385cm^3 of water.

    The volume of the cylindrical tank is V=\pi 5^2 \times h=25\pi h where h is the height.

    Then we substitute in 385: 385=25 \pi h

    h=\frac{385}{25 \pi}

    h=\frac{77}{5 \pi}cm
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  3. #3
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    Hehe thanks!! But I just have a few more questions

    1) \frac{88}{(77)(66)} has the same value as
    a) \frac{0.88}{(7.7)(6.6)}
    b) \frac{0.88}{(0.77)(0.66)}
    c) \frac{8.8}{(7.7)(6.6)}
    d) \frac{8.8}{(0.77)(0.66)}

    2) If \frac{2}{5}x=16-\frac{2}{3}x, then \frac{5x}{2x+10} equals

    Just two more Hope you could help me man.
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  4. #4
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by Rocher View Post
    Hehe thanks!! But I just have a few more questions

    1) \frac{88}{(77)(66)} has the same value as
    a) \frac{0.88}{(7.7)(6.6)}
    b) \frac{0.88}{(0.77)(0.66)}
    c) \frac{8.8}{(7.7)(6.6)}
    d) \frac{8.8}{(0.77)(0.66)}
    You can do this by hand with a little inspiration, but you can do this very easily with a calculator.

    \frac{88}{77 \cdot 66} = \frac{88}{77 \cdot 66} \cdot \frac{\frac{1}{100}}{\frac{1}{100}}

    = \frac{\frac{88}{100}}{\left ( 77 \cdot \frac{1}{10} \right ) \left ( 66 \cdot \frac{1}{10} \right )}

    = \frac{0.88}{7.7 \cdot 6.6}

    -Dan
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  5. #5
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by Rocher View Post
    2) If \frac{2}{5}x=16-\frac{2}{3}x, then \frac{5x}{2x+10} equals
    The simplest idea here is to solve for x and plug x into your expression:

    \frac{2}{5}x=16-\frac{2}{3}x

    \left ( \frac{2}{5}x \right ) \cdot 15 = \left ( 16-\frac{2}{3}x \right ) \cdot 15

    6x = 240 - 10x

    16x = 240

    x = \frac{240}{16} = 15

    So
    \frac{5x}{2x+10} = \frac{5(15)}{2 \cdot 15 + 10} = \frac{75}{40} = \frac{15}{8}

    -Dan
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