Math help? Concurrent schedules of reinforcement

[RabbitWho]

In concurrent schedules of reinforcement [a mouse has a choice between pressing a button to get sugar or pressing it to get cheese], the choice or preference between the two alternative response is calculated by the relative rate of response and is represented by the quotient:

Ra
___
Ra + Rb

Where Ra and Rb represent the rate of response in option a and option b, respectively.
[I guess they mean option a is pressing the sugar and b is the cheese]

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Does this make sense to anyone? I don't understand why you divide those things into each other or what sort of use the answer you get could possibly be.

Let's say he presses the button for sugar 5 times and for cheese twice

5/(5 + 2)
=0.7

Ok now he presses the button for sugar twice and for cheese 5 times.
2/(2+5)
=0.1

What the goose use is that to anyone!?

Is there something missing? Should it say something like "anything over .5 means a is preferred and anything under .5 means b is preffered? Would that make sense?

Maybe it's just so obvious they can't be bothered to mention it .

I translated this, here's the original, in case i messed up:
Volviendo a los programas concurrentes, la elección o preferencia entre las alternativas de respuesta se
calcula por la tasa relativa de respuesta y se representa por el cociente:
Donde Ra y Rb representan, respectivamente, la tasa de respuesta en la alternativa a y la tasa de respuesta
en la alternativa b.