Continuity and conservation of mass
For steady incompressible flow, volumetric flow rate is constant: Q = Av.
When a pipe narrows, the average flow velocity increases. A pipe with area 0.02 m² carrying water at 2 m/s has Q = 0.04 m³/s. If its area reduces to 0.01 m², velocity becomes 4 m/s.
Check your units before substituting values.