Following formula shows the veloctiy, vms-1, at time t seconds for the first 8 seconds of motion:

v=(60t^2-5t^3)/64

Find the acceleration when t=8

I know you have to differentiate and i got 120t-15t^2

However, the mark scheme says to reduce the mark if 64 is left off - why would you leave 64 on?

Equally when I intergrate the formula, apparently 64 is not 64t but just 64, why?

Thanks Very Much