How can find out the time taken to reach r/2 from the other end of tunnel . Please answer quickly with explanation

we can find it by using simple mechanics . earth radius is 6400km so we will find time to caver a distance of 6400 km under an acceleration of 9.8m/s² then we will add this time to the time taken to cover a distance of 3200 km under retardation of 9.8m/s² then we will get 28.2 min as answer..

i hope this will help you ..

thanks and regards

Adarsh