Machine A produces 315 envelopes in 1 hour. Machine B produces envelopes 1.3 times faster than machine A.

To model the situation with a rational equation, let's first determine the production rates of both machines.

1. **Production Rate of Machine :
Machine A produces 315 envelopes in 1 hour. Thus, its production rate is:
\[
R_A = \frac{315 \text{ envelopes}}{1 \text{ hour}} = 315 \text{ envelopes/hour}
\]

2. **Production Rate of Machine B**:
Machine B produces envelopes 1.3 times faster than Machine A. Therefore, its production rate is:
\[
R_B = 1.3 \times R_A = 1.3 \times 315
\]
Calculating that gives:
\[
R_B = 1.3 \times 315 = 409.5 \text{ envelopes/hour}
\]

Now, let \( x \) be the number of hours that both machines operate. The number of envelopes produced by each machine can be expressed in terms of \( x \):

- Envelopes produced by Machine A in \( x \) hours: \( 315x \)
- Envelopes produced by Machine B in \( x \) hours: \( 409.5x \)

If you want to create a rational equation to express the total number of envelopes produced by both machines after \( x \) hours, you can sum their outputs:

\[
\text{Total envelopes} = 315x + 409.5x
\]

This can also be simplified for further analysis if necessary, but if you're looking for a rational expression to model their combined output in terms of time, the equation doesn't need simplification.

The rational equation that models the situation is:
\[
\frac{\text{envelopes produced by A}}{x} + \frac{\text{envelopes produced by B}}{x} = \text{total envelopes}
\]

So, effectively, the fundamental relationship or equation you would be interested in, after substituting the expressions for the envelopes produced, is:
\[
\frac{315x + 409.5x}{x} = \text{Total envelopes}
\]

Please clarify if you need a different type of rational equation or have specific constraints in mind.