A sum or equation in a dream is read as a waking problem that seems to demand a single right answer. Struggling with it suggests pressure to work things out; solving it, a sense of clarity.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.