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Balance the cookies, or balance the variables
The idea for this problem sequence came from a problem in a Japanese textbook.
- At the bake sale, cookies are sold by weight. All apple
cookies (A) weigh the same. All banana cookies (B) weigh
the same (but are a different weight than the apple
cookies). And all chocolate cookies (C) weigh the same
(and are a different weight than the apple and banana
cookies). The cookies can be balanced in the following way:
How many chocolate cookies will it take to balance one
apple cookie?
- Write equations for the left and right balances in problem
one.
- Provide both algebraic expressions and corresponding
pictures for every step of your solution to problem one.
Hints
Hint to problem 1. Substitute some cookies with other cookies
that balance them.
Answers
- One apple cookie balances three chocolate cookies.
-
- 3B = 5A
- B = A + 2C
-
3(A
+
2C)
=
5A
6C
=
2A
Solutions
- From balance 2 (on the right side) we know that one
banana cookie is balanced by one apple cookie and two
chocolate cookies. So, if we substitute all three banana
cookies for apple and chocolate cookies in the first balance
we’ll get:
You
can
substitute
cookies
that
balance
each
other
in
a
number
of
different
ways.
This
solution
presents
one
of
the
possible
ways.
Remove 3 apple cookies on each side:
We see that 2 apple cookies balance 6 chocolate cookies,
or one apple cookie balances three chocolate cookies.
-
- 3B = 5A
- B = A + 2C
-
3(A
+
2C)
=
5A
6C
=
2A
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