Answer
She is 340 m away.
Work Step by Step
She is far away, so when you hear the sound of the hammer blows "exactly synchronized" with the blows you see, the sound you hear each time must be be from an earlier hammer blow.
When you see her stop hammering, and you hear one last strike 1 second later, that sound is due to the very last hammer blow that you saw. It took 1 second to reach you.
$$d = speed \times time = 340 \frac{m}{s} \times (1 s) = 340 m$$
Had you been paying better attention, you would have noticed that her very first hammer blow was silent, and only her second, third, etc. blows were synchronized with the sounds of hammer blows.
