One end of a horizontal oil pipe is closed using a valve and the reading of the barometer attached to the pipe is $2\times {10}^5 \mathrm{Pa}$. When the valve in the pipe is opened the reading of the barometer falls to ${10}^5 \mathrm{Pa}$. The velocity of oil flowing through the pipe will be  (density of oil = 800 kg m^-3) in ms^–1