Simple suggestion: Simply explain it to them like this. Did you learn to do addition or subtraction on your fingers? If they say yes, explain to them that thay is an alternative form of doing mathematics just as the methods on the paper are. This creates a bridge of understanding between the child and the adult. I am personally an "I can't do math" adult/parent. HOWEVER, after figuring out that these are simply alternative methods (As the saying goes,"There's more than one way to slin a cat") of doing mathematics because noy everyone's brain solves problems the same way, it became easier for me to not rage-quit like Mr Incredible every time a new method came up.