for the first 6 trips she walked 3.2 kilometers and for the next 6 she made another 3.2 which would be 6.4 kilometers for 12 trips now she made 3 more trips which is half of the amount she originally did so it would be half of the distance also so 3.2 divided by 2 is 1.6 kilometers so for the last 3 trips she walked 1.6 kilometers. now add up all the distance she walked so 6.4 kilometers for the 12 trips plus the 1.6 for the other 3 trips is 8 kilometers in total. she walked 8 kilometers after 15 trips.