A car is traveling on a straight, level road under wintry conditions. Seeing a patch of ice ahead of her, the driver of the car slams on her brakes and skids on dry pavement for 50 m , decelerating at 7.5 m/s2 . Then she hits the icy patch and skids another 70 m before coming to rest.
A) If her initial speed was 78 mi/h , what was the magnitude of the deceleration on the ice?