The next one is 16 probably
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,

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.
2:15+2:31=4:46
4:46+1:16=5:62=6:02
6:02+2:56 =8:56pm or 20:56
The answer of the equation is 18.