Water flows through a horizontal pipe whose internal diameter changes from 4 cm at one end to 2 cm at the other end. If the speed of water at the wid…

By the equation of continuity, $A_1 v_1 = A_2 v_2$. Since area is proportional to the square of the diameter ($A \propto d^2$), we have $(d_1^2) v_1 = (d_2^2) v_2$. Substituting the values: $(4)^2 \times 2 = (2)^2 \times v_2 \implies 16 \times 2 = 4 \times v_2 \implies v_2 = \frac{32}{4} = 8\text{ m/s}$.