She has run 21 miles already. Let t be the number of additional miles added on.
In total, she has run t+21 miles
This is going to be set greater than 38 since "Ann will run more than 38 miles"
So we have this inequality
t+21 > 38
we solve for t by subtracting 21 from both sides
t+21 > 38
t+21-21 > 38-21
t+0 > 17
t > 17
The final answer is t > 17
which means that the possible additional number of miles she could run is anything larger than 17. So t = 18 is one possibility.
