How would calculate the exchange rate of Bitcoin GBP

Question

I've got an API that returns the value of: USD/Bitcoin & USD/GBP.

{"USDBITCOIN":"4251.27", "USDGBP":"0.758659"}

To calculate the exchange rate of GBP/Bitcoin, I simply divide the the value of the GBP by Bitcoin, because they are both USD based.

How would calculate the exchange rate of Bitcoin/GBP? It may be really simple, but it's really baffling me. I've got the below to calculate

app.js

xOfy(unit, value) {
  return unit / value;
}

yOfX(unit, value) {
  return unit * value;
}

xOfy(gbp, bitcoin)
// 0.00018

yOfx(bitcoin, gbp)
// 3225.26