Question

There are 10 identical coins and each one of them has ‘H’ engraved on its one face and ‘T’ engraved on its other face. These 10 coins are lying on a table and each one of them has ‘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 total number of attempts in which the ‘T’ faces of all the 10 coins can be brought to be the upper faces?

(a) 4 (b) 7 (c) 8 (d) Not possible

Answer 1

(a) On the first attempt four coins are overturned. Now, six coins are left.

In the next turn, four more are overturned. Now only two would be left. We take one more from the left over two coins and any three from the previously turned ones. Finally, the leftover coin and the three coins from the presiding step which have already been turned twice can be overturned. Thus, in four attempts, one can complete the process.