Soulver lets you reference previous lines' answers in your calculations, kind of like a spreadsheet. You can go back and edit a previous line, and any lines that refer to will be recalculate automatically.
1. Drag in the answer from the line you want to reference:
2. Type an operator (like +) immediately after typing return to start a new line:
You can also insert a reference to the previous line using the keystroke ⌘\.