You could add/subtract/multiply/divide with stones quite happily. And indeed, that's how it's introduced to kids at infant school, though probably with a Health and Safety stone substitute. So, you could start like that and make the numbers bigger and bigger and more inconvenient. read more
If not, multiply them together (and balance the numerators accordingly) Add or subtract using the numerators, keeping the denominator the same. For an adding or subtracting task, you need to make "common denominators". The hardest thing about common denominators is trying to say it. read more