Consider the set of all fractions of the form \frac{2^m}{3^n}, where m and n are integers such that 0 \le m \le n \le 3. For example, \frac{1}{9} and \frac{8}{27} would be included in this set, but not \frac{32}{81}. What is the sum of all the numbers in this set?

1 answer

We need to consider all fractions of the form \( \frac{2^m}{3^n} \), where \( m \) and \( n \) are integers satisfying \( 0 \leq m \leq n \leq 3 \).

Let's enumerate all the possible values of \( m \) and \( n \) under the constraints given.

  1. If \( n = 0 \):

    • \( m = 0 \) (since \( 0 \leq m \leq 0 \)): \( \frac{2^0}{3^0} = 1 \)
  2. If \( n = 1 \):

    • \( m = 0 \): \( \frac{2^0}{3^1} = \frac{1}{3} \)
    • \( m = 1 \): \( \frac{2^1}{3^1} = \frac{2}{3} \)
  3. If \( n = 2 \):

    • \( m = 0 \): \( \frac{2^0}{3^2} = \frac{1}{9} \)
    • \( m = 1 \): \( \frac{2^1}{3^2} = \frac{2}{9} \)
    • \( m = 2 \): \( \frac{2^2}{3^2} = \frac{4}{9} \)
  4. If \( n = 3 \):

    • \( m = 0 \): \( \frac{2^0}{3^3} = \frac{1}{27} \)
    • \( m = 1 \): \( \frac{2^1}{3^3} = \frac{2}{27} \)
    • \( m = 2 \): \( \frac{2^2}{3^3} = \frac{4}{27} \)
    • \( m = 3 \): \( \frac{2^3}{3^3} = \frac{8}{27} \)

Now, we list all the fractions we have computed:

  • From \( n = 0 \): \( 1 \)
  • From \( n = 1 \): \( \frac{1}{3}, \frac{2}{3} \)
  • From \( n = 2 \): \( \frac{1}{9}, \frac{2}{9}, \frac{4}{9} \)
  • From \( n = 3 \): \( \frac{1}{27}, \frac{2}{27}, \frac{4}{27}, \frac{8}{27} \)

Next, we add all the fractions together:

\[ 1 + \frac{1}{3} + \frac{2}{3} + \frac{1}{9} + \frac{2}{9} + \frac{4}{9} + \frac{1}{27} + \frac{2}{27} + \frac{4}{27} + \frac{8}{27} \]

First, sum the fractions with common denominators.

  1. For denominators \( 1 \), \( 3 \), \( 9 \), and \( 27 \), we can express all terms with a common denominator of \( 27 \):
  • \( 1 = \frac{27}{27} \)
  • \( \frac{1}{3} = \frac{9}{27} \)
  • \( \frac{2}{3} = \frac{18}{27} \)
  • \( \frac{1}{9} = \frac{3}{27} \)
  • \( \frac{2}{9} = \frac{6}{27} \)
  • \( \frac{4}{9} = \frac{12}{27} \)
  • \( \frac{1}{27} = \frac{1}{27} \)
  • \( \frac{2}{27} = \frac{2}{27} \)
  • \( \frac{4}{27} = \frac{4}{27} \)
  • \( \frac{8}{27} = \frac{8}{27} \)

Adding these all together gives:

\[ \frac{27}{27} + \frac{9}{27} + \frac{18}{27} + \frac{3}{27} + \frac{6}{27} + \frac{12}{27} + \frac{1}{27} + \frac{2}{27} + \frac{4}{27} + \frac{8}{27} \]

Calculating the numerator:

\[ 27 + 9 + 18 + 3 + 6 + 12 + 1 + 2 + 4 + 8 = 90 \]

Thus, we have:

\[ \frac{90}{27} = \frac{10}{3} \]

Therefore, the sum of all the numbers in this set is:

\[ \boxed{\frac{10}{3}} \]