The replies suggest a need to separate mental math from mental arithmetic. I can't do mental arithmetic despite a lifetime of trying to develop the ability, but

The replies suggest a need to separate mental math from mental arithmetic. I can't do mental arithmetic despite a lifetime of trying to develop the ability, but still have no trouble with the mathematic intuition based on expected costs described in other comments. It's easy enough to round things to 0 or 5 and sum it up for comparison even if doing the actual numbers in my head just won't happen.