I am not sure if this is the right place to post this or if should e-mail Turing Tumble direct. So, after my daughter and I finish the task we like to see if we had come up with a different solution then shown in the back of the book. With challenge #15 you had 11 bits and 2 crossovers to your disposal and we used all parts to make it work. However your solution shows 12 bits and 1 crossover as the solution. We have a Kickstarter edition and this might have been corrected in the one that is available in stores.
3 of the 12 ramps in the solution are from the starting setup. Only the remaining 9 come from the pool of 11 additional ramps available to add in creating the solution.
I suspect you’ve come back to the game recently after a break, and forgotten some of the details without realising - it might be worth flipping through the earlier pages to refresh your memory when you come back to the game after a period away.