Just wanted to add this tip, for Challenge 3, as I couldn't find it myself either when I was looking for it. Just reach as high as possible in terms of income then be on the lookout for when the multiplication is super high. When you see a very high number (into the millions), then time travel exactly at that point. I was stuck in Challenge 3 for 2 days and I thought of this trick and immediatelly finished it after. Hope it helps others as well!