Here's my take.....
The ant never gets to the end of the string
Each second, the distance that the ant covers is t inches
And each second, the length of the string (in inches) = 12 + 12 t
So.....the ratio of the distance that the ant has traveled in t seconds to the string length at that time is just
t
________
12 t + 12
As t approaches some infinite number of seconds, this ratio converges to
t 1
_______ = ____
12 t 12
Therefore, the ant can never travel much more than 1/12 the string length as t approaches infinity