We will have that the rate is the following:
90:2
90 beats every two minutes, and can be simplified as follows:
45:1
So, the simplified rate is 45:1.
***
We have that the relationship is 90 beats every two minutes, and that can be seen as a division:

And we can see the rate as the components of division 90:2.
When we operate, we obtain the ratio:

So, the ratio can be seen as 45:1.