Hands-on math!

Washington State Math Olympiad
Hints and Solutions
2009 Grade 6 Algebra

Problem
Solution
1) Jane bought a 36 inch rope of licorice to share with her brother Andy. She gave Andy a piece that is three inches more than half the length of her piece. How long is Andy's piece of licorice? Let J = length of Jane's piece.
1. Then the expression for Andy's piece is
    "3 inches more than half of Jane's piece" = J/2 + 3.
2. Make an equation that adds Jane's and Andy's pieces and sets them equal to the whole length =
  J + (J/2 + 3) = 36.
  (3/2)J = 33
  3J = 66
  J = 22
3. Solve for Andy's piece =
    (22/2) + 3 = 11 + 3 = 14 inches
2) Mr. Jones takes three taxi cab rides with the same cab driver. The table shows how many miles he traveled and what the taxi fare was. Use the information to decide what the base fare without tip is for a trip that is 6 miles long.
1. Take the difference between the 4 and 2 mile trips and divide by the distance difference = ($14.50 - $8.50)/2 = $3. This is the charge/mile over the base amount.
2. Using the 4 mile trip, subtract the charge/mile to get the base =
    $8.50 - 2 X $3 = $2.50.
3. Apply this to the 6 mile trip =
    $2.50 + 6 x $3 = $20.50
3) Pictured is a sequence of growing chairs. The first chair is made of 6 squares. How squares are in the 20th chair in the sequence? 1. Write out the sequence of numbers of squares in each chair:
    6     10     14 ...
2. Determine the increase in the number of squares per chair = 4 cubes.
3. Use the formula for the Nth term of an arithmetic sequence to determine the number of squares in the 20th chair =
    An = A1 + D (n - 1) where:
    An = Nth term (the term you are looking for)
    A1 = First term
    D = Number added to each term
    A20 = 6 + 4 (19) = 82 chairs

Problem
Solution
4) Over five months, Mrs. Simons wishes to give $1,000 to her favorite charity. Her plan is to give them more each month. The last month or fifth month will be the biggest donation. The fourth month she will give 1/10 what she gives the fifth month. The third month she will give 1/10 what she gives the fourth month. If this pattern continues how much will she give the last month to the nearest dollar? Let X be the amount she gives in the 5th month.
1. Then the equation is

    X + (.1)X + (.01)X + (.001)X + (.0001)X = 1000.

2. Solve for X :
    X + (.1 + .01 + .001 + .0001) X = $1000
Combine terms:
    X + .1111 X = $1000
    1.1111 X = $1000
    X = (1000 / 1.1111) = $900.
5) To discourage guessing on multiple choice tests, Mr. T. has instituted a new way of grading: 4 points for every correct answer and lose 1 point for every wrong answer. There are 15 questions on the test Sally takes and she scores a 5. How many did she get right and how many did she get wrong? Method 1:guess-and-check
Guess #  # right  #wrong  Total 
110535
24114 x 4 -11 = 5

Method 2: Use 2 equations:
    Use C for # correct and W for # wrong.
    Write the equation for the total number of problems
    C + W = 15
    Then, the equation for "4 times the number of correct problems minus the number of wrong answers = 5" is
    4C - W = 5
    Turn the first equation into an equation for W =
    W = 15 - C
    and substitute that value in the second equation and solve:
    4C -(15 - C) = 5
    4C + C = 6
    5C = 20
    C = 4     W = 11