[ u = 4.5,\text{m/s},\quad a = 2.5,\text{m/s}^2,\quad v = 20,\text{m/s}. ]

[ v = u + at \implies 20 = 4.5 + 2.5t \implies 15.5 = 2.5t \implies t = \frac{15.5}{2.5} = 6.2. ]

Answer: 6.2 seconds