The formula to calculate the fundamental frequency in a closed pipe is given by
fc=4l1v...(1)
And, that in an open pipe is given by
fo=2l2v...(2)
Given that
∣fc−fo∣=7...(3)
From equations (1), (2) and (3), it follows that
4×150v−2×350v=7⇒600cmv−700cmv=7⇒6mv−7mv=7⇒v(421)=7⇒v=42×7=294ms−1