Rational numbers are numbers that can be expressed as a fraction of two integers, where the denominator is not equal to zero. In this case, we have the expression "5 upon 9 multiply minus 7 upon 15". Let's break it down and solve it step by step.

Step 1: Simplify the expression "5 upon 9 multiply minus 7 upon 15" by performing the multiplication and subtraction separately.

5 upon 9 multiply minus 7 upon 15

Step 2: Perform the multiplication of the fractions by multiplying the numerators together and the denominators together.

(5 * -7) upon (9 * 15)

Step 3: Simplify the numerator and denominator separately.

-35 upon 135

Step 4: Simplify the fraction by finding the greatest common divisor (GCD) of the numerator and denominator, and dividing both by it.

-35 and 135 have a GCD of 5.

-35 upon 135 can be simplified to -7 upon 27.

Final Answer: The expression "5 upon 9 multiply minus 7 upon 15" simplifies to -7 upon 27.

Summary:
- Rational numbers are numbers that can be expressed as a fraction of two integers.
- The expression "5 upon 9 multiply minus 7 upon 15" is a rational number expression.
- To solve the expression, we perform the multiplication and subtraction separately.
- After simplifying the expression, we get -7 upon 27 as the final answer.

-7/27