How does the speed and time calculator work?

Question

JavaScript

It is necessary to implement a similar calculator, as here: www.countcalculate.com/cars-and-speed/time-savings...
I have such a question, when setting speed values, choosing a metric, etc., then the item "how many minutes will I arrive before" is considered correct, if you look at the Answer block, then this is the 3rd paragraph.
You will reach your destination 100.0 minutes earlier , that is in 200.0 minutes . (for example, this is understandable, the first time and the second time).
But I don't understand how the first two values ​​are obtained:
You gain 30.0 minutes every hour by increasing the speed from 40.0 to 60.0 Miles per hour (mph) . At 60.0 Miles per hour (mph) it takes 1.0 minutes to travel one mile
I highlighted incomprehensible moments in italics, if you put miles in select , then the second paragraph correctly considers that 60 mil per hour is 1 mile per minute, this is understandable. But when I set kilometers, it turns out some kind of crap. At 60.0 Kilometers per hour (km/h) it takes 10.0 minutes to travel one metric mile . . Ne clear!
Well, the first value of 30 minutes is not clear , where does it come from?
Initial speeds, well, for example 40 and 60 .

Answer 1

You gain 30.0 minutes every hour by increasing the speed from 40.0 to 60.0 Miles per hour (mph) .

This means that at a speed of 60 you will travel 60 miles in an hour.
At a speed of 40 you will cover those 60 miles in 1 hour and 30 minutes. These 30 minutes are You gain 30 minutes.