Verrrrrry interesting !
Acceleration = (change in speed) / (time for the change)
The car's acceleration is (60 mph) / (3.8 sec) = (60/3.8) mile/hr-sec .
Final speed = (original speed) + (acceleration · time)
= (30 mi/hr) + (60/3.8 mi/hr-sec)·(6 sec)
= (30)mi/hr + (360 / 3.8)mi/hr
= 124.7 mph .