I like doing maths in my head

Ppl like how do you do those big multiplications in your head? Take 13 times 17. 13 times 10 is 130 13 times 7 is 70 plus 21 so 91. And 91 plus 130 is 231. So 13 times 17 is 231. 🤷🏼 It entertains myself.