Hello there!
So, if this turtle is only going 2.5 miles per hour. This would mean that (EACH) hour, this turtle only arrived 2.5 miles. So, we do,
[tex]\boxed{10.0 \left \{ {{2.5+2.5=5.0} \atop {2.5+2.5=5.0}} \right.}[/tex]
We would have to count how many ("2.5's" they are above.)
We count only 4.
I would take this turtle 4 miles to swim 10 kilometers.