If a car can go from 0 to 60 mi/hr in 8.0 seconds what would be its final speed after 5.0 seconds if its starting speed were 50 mi/hr?

Let’s use Newton’s Laws of Motion to determine a few things…
First the of the car:
##a = (v_f – v_i)/t## where ##v_f = 26.82 m/s; v_i = 0; t = 8.0 s##
##a = (26.82 – 0)/8.0 = 3.35 m/s^2##
Now to determine the final speed when ##v_i = 22.35 m/s; t = 5 s; a = 3.35 m/s^2##
##v_f = v_i + at##
##v_f = 22.35 + 3.35*5 = 39.1m/s##
The same problem in English units of miles per hour:
Acceleration of the car from 0mi/h to 60mi/h in 8.0s:
##a = (v_f – v_i)/t## where ##v_f = 60mi/h; v_i = 0; t = 8.0 s##
##a = (60mi/h – 0)/8.0s = 7.5mi/h/s##
Final speed when initial velocity is 50mi/h:
##v_i = 50mi/h##; ##a = 7.5mi/h/s##; ##t = 5.0s##
##v_f = v_i + at##
##v_f = 50mi/h## + (##7.5mi/h/s##)(##5.0s##)
##v_f = 50mi/h + 37.5mi/h = 87.5mi/h##