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Very nice! \(v\) is the final velocity, \(u\) is the initial velocity and \(t\) is the time. As Jerry was initially travelling at \(2.5 \, \mathrm{m/s}\), \(u = 2.5 \, \mathrm{m/s}\). As he finally reached a velocity of \(5 \, \mathrm{m/s}\), \(v = 5 \, \mathrm{m/s}\). And, as he took a time interval of \(2 \, \mathrm{s}\) for all this, \(t = 2 \, \mathrm{m/s}\).
Because we know \(v, \,u\) and \( t\), we can calculate \(a\), the acceleration. Using \(v = u + a*t\), \(a = \frac{v - u}{t} = \frac{5 - 2.5}{2}\). From this, we get \(a = 1.25 \, \mathrm{m/s^2}\).