text: Put the first derivative equals to zero, and find t\n\nf(t) = 98.7 t Exp(-.31 t)\n\nf'(t) = 98.7 t (-.31) Exp(-.31 t ) + 98.7 Exp(-.31 t)\n\nPut f'(t) =0 -->\n\n0 =98.7 t (-.31) Exp(-.31 t ) + 98.7 Exp(-.31 t)\n\n0 = Exp(-.31 t )( -.31 t + 1 )\n\nEither Exp(-.31 t) =0 ---> illegal\nOr\n-.31 t + 1 =0 --> t = 3.2258