1. The problem involves a plane flying with a wind blowing from the West at 15 km/h and the plane's airspeed is 90 km/h. 2. We want to find the resultant velocity of the plane considering the wind. 3. Assume the plane flies directly North (for example) with speed $90$ km/h. 4. The wind blows from the West to the East at $15$ km/h. 5. The resultant velocity vector $\vec{v}$ is the vector sum of the plane's velocity $\vec{v_p} = (0, 90)$ and the wind velocity $\vec{v_w} = (15, 0)$. 6. Calculate the magnitude of the resultant velocity: $$ |\vec{v}| = \sqrt{15^2 + 90^2} = \sqrt{225 + 8100} = \sqrt{8325} \approx 91.3 \text{ km/h} $$ 7. Calculate the direction angle $\theta$ east of north: $$ \theta = \tan^{-1}\left(\frac{15}{90}\right) = \tan^{-1}(\frac{1}{6}) \approx 9.46^\circ $$ 8. So the plane's ground velocity is approximately $91.3$ km/h at $9.46^\circ$ east of north.