Dreaming of calculating multiplication does not mean that the dreamer has a good fortune in the near future, suggesting that the external pressure is a bit great. The answer is for reference only, please don't be blindly superstitious.
Dream, a common word in modern Chinese, was first seen in Oracle Bone Inscriptions. Six books belong to literacy. The basic meaning of the word dream is an illusion in sleep, only waking up after a dream, but not completely getting rid of it. The extended meaning is a metaphor for fantasy or wish, like a dream.