Sinusoidal  waves 5.00 cm in a amplitude are to be transmitted along a string having a linear mass density equal to $4.00\times {10}^{-2}kg/m$. If the source can deliver a maximum power of $90W$ and the string is under a tension of $100 N$, then the highest frequency at which the source can operate is (take ${\pi }^2=10$):