There are 10 identical coins and each one of them has ‘H’ engraved on one face and ‘T’ engraved on the other face. These 10 coins are lying on a table and each one of them has the ‘H’ face as the upper face. In one attempt, exactly four (neither more nor less) coins can be turned upside down.
What is the minimum number of attempts in which the ‘T’ faces of all the 10 coins can be brought to be the upper faces?
