B shows her getting closer to home, immediately turning round, gives time for finding book then riding all the way home at a constant speed
11c-4c = 7c
5c+2c= 7c
3c+7c= 10c
11c-4c= 5c+2c =A
Multiplying both sides by gives
so that substituting and hence gives the linear ODE,
Now multiply both sides by to get
so that the left side condenses into the derivative of a product.
Integrate both sides, then solve for , then for :