use a calculator, on line or handheld. TI84 does it all and more than you'll ever need. About $100+ lately. Endless online calculators will similarly solve more problems than you'll ever see.
or use a Chinese abacas, like the math genius girl named Mei did in a Jason Statham movie. She had a memory like Big Bang's Prof. Sheldon Cooper, infallibly accurate. She hated math, but was better than anyone, ever.
but more normal people can do some mental math, not on a par with the genius though, but still some calculations, minimum learn the mutliplician tables. 6x9=54 doesn't require marbles, pen or paper. IF you forget 6x9=54, but remember 6x8=48, just add 6 to 48 mentally, 6x9= 48+6=54
48+6 can be done mentally. 8+6=14, that's 4 and carry the 1 = 14+40= 54
