I also perform mental arithmetic like this. For the sake of more examples on how I do math, I often factorize numbers when multiplying (not necessarily prime factorization). I often also treat numbers as binomials when squaring them, i.e., in order to square 26, I'll do (20 + 6)^2, and if you remember your binomial theorem, (a + ^2 = a^2 + 2ab + b^2. This means that 26^2 = (20 + 6)^2 = 400 + 240 + 36 = 400 + 276 = 676.