Pose a word problem base on this solution

My mom has trouble understanding math... I decided to give her an easy equation. 3/6 km+1/5km = 15/40 +8/40. She asked me what I did. I told her that I multiplied both equations to get a denominator of 40. Then she said "what about those top ones" "oh... well" I said " whatever I multiplied to the bottom I have to do to the top!" Anyway back to the problem. Well... somehow all of this equals 23/40. This is because when you add 15 + 8 that equals 23! So wait a sec what is this on the bottom she asked. well... I am unsure why don't you tell me. She said that the 4 needed to have been added to it.. but the four was too much so they took away one. Good enough! The answer is 3 and 17/40