hi nicboo

Welcome to the forum.

This is how you can devise your own formula for this:

Think of a very easy example. Say you want to pack 1000 envelopes and you know you can do 200 per hour.

That's two hundred in the first hour, another two hundred in the second hour, ............, another two hundred in the fifth.

Can you see this will take 5 hours?

So what is the connection between the three numbers ?

So now put words in place of the numbers:

This time will be in hours. Times by 60 to change to minutes.

Bob