Lyn's speed is 3 miles per hour.
That is distance travelled in 60 minutes = 3 miles
So, distance travelled in 25 minutes = 3 * 25 / 60 miles
= 25/ 20 miles
= 1.25 miles
New speed of Lyn is 5 miles per hour
That is, for travelling 5 miles, time required = 60 minutes
So, for travelling 1.25 miles, time required = 60 * 1.25 / 5 minutes
= 60/ 4 minutes
= 15 minutes.
It will take her 15 minutes, if she runs at a speed of 5 mph.
Hope this helps ..!!