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##