When an open organ is dipped in water upto half of its height, then...

Frequency of open organ pipe is double than that of closed pipe. If frequency of open pipe $=n$ $\therefore$ Frequency of closed pipe $=\frac{n}{2}$ On inserting an open pipe into water upto half of its height, it will behave like a closed pipe of half length and frequency is inversely proportional to the length of air column. Hence, frequency of closed pipe will become double $=2 \times \frac{n}{2}=n$