a liquid flowing with a speed v through a horizontal pipe of cross sectional area A enters into another pipe of double the area of cross section. now the speed of liquid is-

In stesdy state total volume ( and hence mass ) should be conserved.

volume = A.vΔt.

where A : area of cross section

        vΔt : length travelled in time Δt.

since

initial volume= final volume

it implies

since A₁/A₂ =  1/2

therefore v₂ = v1/2

                  = v/2