OMG I messed up and I can't edit. Turtle doesn't run at 0.4 mph; Turtle runs at 1 mph.
I didn't copy the question like I usually do, and I wasn't glancing up at it as I worked through.
Evverywhere I referred to Turtle's speed as 0.4 mph above, change that to 1 mph.
So the calculation should be D = S • T
0.4 mile = (12 miles/hour) • T
(0.4 mile) / (12 miles/hour) = T
T = 0.0333333 hour (i.e., 1/30 of an hour = 2 minutes )
Check the answer:
Achilles - (1/30 of an hour) • (13 miles/hour) = 0.4333 mile
Turtle - (1/30 of an hour) • (1 mile/hour) = 0.0333 mile
So, we confirm that at the end of 1/30 of an hour, Achilles is 0.4 mile ahead of Turtle.
And I'm sorry for the error in my original answer;
.