The reading of pressure metre attached with a closed pipe is 4 . 5...

The formula to calculate the change in pressure can be written as

${P}_{1} - {P}_{2} = \dfrac{1}{2}ρ{v}^{2} . . . \left(1\right)$

From equation (1), it follows that

$4 . 5 \times {10}^{4} - 2 . 0 \times {10}^{4} = \dfrac{1}{2} \times {10}^{3} \times {v}^{2} ⇒ 2 . 5 \times {10}^{4} = \dfrac{1}{2} \times {10}^{3} \times {v}^{2} ⇒ {v}^{2} = 50 ⇒ v = \sqrt{50}$

Hence, $V = 50$.