1ya we can simply differentiate with time and get the answer
if we differentiate, we get dv/dt = -v2duu2dt
if u substitute the values u will find that the answers are almost the same
the one that u get through differentiation is more correct(it gives instantaneous velocity)
the answers in the textbook are average velocities(which is clearly mentioned there)
they just found out the change in u over 1 sec and hence the change in v,
then v(avg over 1 sec ) = change in v/ 1 sec
