Answer: four blows of hammer
Explanation:
The first blow of hammer drove the 3cm nail into wood by 1.25cm leaving 1.75cm of the nail
3/5 or 0.6cm of the nail was also driven into wood by the second blow of hammer leaving 0.7cm of the nail
The third is also drove 0.6cm of the nail in leaving 0.1cm
since 0.1cm is not up to 0.7cm,it will definitely be driven in completely by the fourth blow of hammer.