You divide by 60.
Step by step:
0.25 miles / minute
We want to get rid of 'minute' and replace it with 'second.'
Remember how we can multiply by 1 and it doesn't change anything? That means we can * (1 minute / 60 seconds) because (1 minute / 60 seconds) = 1. We can do this:
(0.25 miles / minute) * (1 minute / 60 seconds)
See how we have (/minute) * (minute)? The minutes cancel out, giving us this:
(0.25 miles / 1) * (1 / 60 seconds)
Now we have miles / seconds. Let's move all of the numbers over to one side like this:
(0.25 / 1) * (1 / 60) * miles/seconds
(don't worry about seconds vs second, they're sort of the same, like 1 second = 1 seconds)
0.25/60 miles / second
about 0.004167 miles / second