Answer:
20 mph
Explanation:
Granny stopped from 50mph,
Her deceleration per unit time is
a = 50 - 0 = 50m/hr^2
This is also the amount of deceleration you experience per unit time (because both cars are identical), therefore your final speed with which you collide with the truck will be,
50 = 70 - v
V = 70 - 50 = 20mph